TheSamba.com :: View topic - running hot, miss fire, timing light, distributor, high rpm

Sun Dec 20, 2015 3:11 pm

I need some help
I will make a time line of work prior to this issue
1) installed new vw10 vacuum advance distributor
Found TDC of course and popped the distributor in. Did not reassure timing with any diagnostic tool
2)rebuilt carb because there was a fuel leak coming from the seal of the intake manifold.
Leak went away and car was running great and crisp (side note it would go 75-80 mph) but sometimes stumbled in 2nd
3) got a timing light and went to set the timing right off the bat it was far advanced (the white mark was left of the notch where the tdc seam is in the case
I light up the mark with the case seam/notch but it would stumble and misfire. I adjust the idle/fuel mixture screw but after 2 days of messing with that went ahead and advanced it again
4) today I got a test light because there was a video on YouTube. It said connect the clip to neg and light to ground (carb) key in on position and turn the distributor when the light is on you're good when it goes out its not. But when I did it it kept staying on. Not sure if I wasn't Turing it enough but I felt like it was enough just didn't want to turn it to much and get to far from where I needed to be

I'm lost at this point. I advanced the distributor close to where it was but a little closer to the crank seam so it could be closer in spec but now the engine gets hotter faster and so far barley misfiring but won't go past 50-60mph
I don't know if I need to advance it more or retard or just go back get it in specs then work on the idle fuel screw and bypass screw (3-4 turns out)

Sun Dec 20, 2015 5:06 pm

You need to time it at 32* BTDC at full advance with a timing light with the vacuum hose disconnected and plugged.

Sun Dec 20, 2015 5:35 pm

^Yes, what he said^
busdaddy wrote: Here's the sermon :D
First you need to determine exactly where TDC is on your pulley and then set the timing at speed like this: Here's my timing for noobs rundown (keep in mind this assumes you have a degree wheel, timing scale or dial advance timing light and know where TDC really is, if you don't understand the marks on your type 1 pulley read this: http://www.thesamba.com/vw/forum/viewtopic.php?t=251672&highlight=stock+pulley+marks ). Or if type 4 see this: http://www.ratwell.com/technical/FindTimingMark.html

If you only have one hose attached to your distributor pull it off, you don't need to cap it (no need on single hose systems), if you have 2 pull them both off but plug the one that was connected to the nipple facing the distributor (retard {not you, that's what the hose does}). Now connect the timing light to the battery and the #1 plug wire according to the timing light instructions (and set it to zero if it's the type with adjustments). Now start the engine and shoot the timing light at the scale and pulley (hold the light in your right hand), see the mark on the pulley?, good. Now using your left hand slowly open the throttle on the side of the carb or throttle body (move it the same way the cable pulls it) and watch the timing mark VS: the scale, the mark on the pulley should start to move to the left, open the throttle a little more and continue until the mark no longer moves to the left any more (yes it's loud, isn't it?), give it a bit more throttle just to confirm the mark is staying put at wherever it stopped (hopefully 28 degrees) and then release the throttle. If it stopped at 28 move on to carb/FI adjustment, if it went past or didn't make it all the way loosen the distributor clamp a little and turn the distributor a few degrees one way or the other (you pick, if it's worse go the other way), repeat until you find the happy spot and don't forget to tighten the clamp when you are done (make sure the distributor is pushed down all the way into the case too). Avoid loose fitting clothing and long hair near spinning fans and belts too, no need for a trip to emergency. Now put the hose back on, pack up the timing light and move on to carb or FI adjustment.
After you get that under control deal with the carb and don't mess with the timing again, if it still runs lousy you have a vacuum leak or carb issues that need to be adressed.

Sun Dec 20, 2015 5:46 pm

Okay thanks for the help

Sun Dec 20, 2015 6:55 pm

You seem to be making some assumptions based on stock parts while you no longer have stock ignition parts.

First question.... what do the marks on your crank pulley represent?? This is not a trick question. Some stock VW crank pulleys have a single "notch". These could represent 5ATDC which was the correct timing for the distributor that came with that pulley. But when you change the distributor that may no longer be the correct timing mark (timing should be set based on the distributor being used). For example, if your timing mark represents 5ATDC and your new distributor is expecting an idle timing of 7BTDC you would be setting your timing 12deg too retarded and should see problems such as lack of power and overheating.
If you are not certain what the timing mark(s) represent... find out. Do not assume anything... well, you can assume is that the crank pulley marks are NOT the correct one for your application, until you confirm it. :D
This thread may help you identify which stock pulley you have:
http://www.thesamba.com/vw/forum/viewtopic.php?t=2...p;start=59

I'm not familiar with the VW10 distributor. What is the correct idle timing for this aftermarket distributor? Have you set the timing per this spec? Correct rpms and with or w/o the vacuum hose connected?

Is your timing light the simple type with just an ON/OFF trigger? Or the more complicated "advanced" timing light with adjustable pulse (dial or digital up/down advance adjustment)? This "advanced" type allows you to use just the TDC timing mark to set any amount of BTDC timing.
If using an "advanced" timing light be sure the adjustment is set to zero to use it as a normal timing light.

If your ignition has a electronic points (I think the VW10 does) you may not be able to use a simple test lamp to set "static timing".
If your engine runs and you have a timing light (and known how to use it), use it to set timing. It is more accurate and the proper way to time modern ignitions.

Sun Nov 02, 2025 11:24 am

Joey wrote: You need to time it at 32* BTDC at full advance with a timing light with the vacuum hose disconnected and plugged.

I am not understanding why you would disconnect the vacuum hose and plug. Why wouldn't you check the timing as you would be driving normally with the vacuum hose connected? Thanks! :)

Sun Nov 02, 2025 11:46 am

When you are talking about SVDA, the vacuum advance is conditional and the centrifugal is the core of the advance, and that is the one you want to set to 30-ish total advance. So you disable the vacuum when testing that.

When you have the vacuum-only distributors of the 1960s, if you want to measure total advance, on THOSE you have to have the vacuum hose connected (and ironically, for idle speed timing setting you often should unhook the vacuum hose to get a stable setting that reflects the static value, this depends on what distirbutor and carburetor you have and how the carb is set up)

Sun Nov 02, 2025 12:22 pm

glutamodo wrote: When you are talking about SVDA, the vacuum advance is conditional and the centrifugal is the core of the advance, and that is the one you want to set to 30-ish total advance. So you disable the vacuum when testing that.

When you have the vacuum-only distributors of the 1960s, if you want to measure total advance, on THOSE you have to have the vacuum hose connected (and ironically, for idle speed timing setting you often should unhook the vacuum hose to get a stable setting that reflects the static value, this depends on what distirbutor and carburetor you have and how the carb is set up)

Thanks Andy! So, would this also apply to Ignition: Electronic w/ Vacuum Advance ? Thanks!

Sun Nov 02, 2025 2:31 pm

The generation of spark does not matter if you're using a timing light and setting total advance. You can do retrofit electronic ignition into pretty much any VW distributor.

If it's an SVDA or DVDA distributor, to check total advance you only do the centrifugal, so with advance hose removed. If it's SVA (vacuum-only) the you HAVE to have the vacuum hose connected.

Mon Nov 03, 2025 11:42 am

glutamodo wrote: The generation of spark does not matter if you're using a timing light and setting total advance. You can do retrofit electronic ignition into pretty much any VW distributor.

If it's an SVDA or DVDA distributor, to check total advance you only do the centrifugal, so with advance hose removed. If it's SVA (vacuum-only) the you HAVE to have the vacuum hose connected.

Thanks Andy. I am not sure which I have. This is a new engine. I will call the builder and find out. :)

Mon Nov 03, 2025 2:18 pm

DanCarr wrote: I am not understanding why you would disconnect the vacuum hose and plug. Why wouldn't you check the timing as you would be driving normally with the vacuum hose connected?
Ideally, you could do this... check timing with everything connected... if you could check/adjust your timing AS YOU ARE DRIVING. This is so you can confirm the ignition timing with the engine under load (eg. climbing a steep hill) and while not under load (eg. cruising at constant freeway speeds). Obviously, this is not practical.

There are four common ignition advance systems used in ACVWs:
Mechanical advance - RPM-based timing advance. This is commonly managed with weights and springs. By varying the weights+springs you control how quickly the advance comes in. Some distributors have a single weight+spring, others have dual-weights+springs. A single weight system typically has a linear advance curve. A dual weight system can have progressive curve that changes at different rates at different rpms.
The downside of a mechanical-only advance is it cannot read the load on the engine and adjust based on that load.
As a general rule with internal combustion enegines... you want to increase mechanical advance as rpms increase up to a max of around 35BTDC (28-32BTDC for ACVWs) at higher rpms (~3500rpms). Above this rpms additional rpm-based timing advance is not needed. Turbulence in the combustion chamber decreases the time it takes to complete combustion, basically advancing the combustion event w/o increasing the ignition advance.

Vacuum advance - Load sensing advance found in the SVDA/DVDA distributors. The vacuum canister is connected to ported or intake vacuum. When an engine is under minimum load, the throttle plate is almost completely closed and the intake vacuum is HIGH. When the engine is under load, the intake vacuum drops and can be near zero measured vacuum (WOT). Vacuum advance connected to intake or ported vacuum senses the vacuum level and increases timing advance as the vacuum increases. This allows additional ignition timing advance when the engine is under low/light load; increasing efficiency, power and fuel economy. Then as the engine load increases (vacuum decreases) the timing is retarded to avoid detonation under load. This system varies the ignition advance automatically in response to engine load.
This vacuum advance is usually paired with mechanical advance... allowing the mechanical advance to increase ignition timing based on engine rpms, with the vacuum system adding extra timing advance when under light load to improve power/efficiency/fuel economy. It automatically retards, reducing/removing the additional timing advance as engine load increases.

Vacuum retard - Idle timing retard to reduce emissions. On later model Beetles, VW added a timing retard system (DVDA) that delayed the ignition event at idle. By delaying the ignition (combustion event) at idle by as much as 12deg, this results is increased heat from combusion still available when the exhaust valve starts to open. This carries more heat into the exhaust to burn off any unburnt fuel. This reduces the hydrocarbons(?) that get pumped out the exhaust pipe.
This is why these DVDA distributors were typically run with an idle ignition timing of 5ATDC. This was basically a static timing of 7BTDC but with the engine running and vacuum retard removing 12deg, the idle timing with the vacuum hoses connected resulted in an idle timing of 5ATDC (7BTDC - 12deg = 5ATDC). The carb needs to be tuned to run with a large amount of ignition timing removed.

Vacuum advance (early distributors - large diameter vacuum canisters). These vacuum advance distributors worked off a different principle. They had no mechanical advance, only vacuum advance. They used much lower levels of vacuum than the later small diameter vacuum advance canisters. As air velocity down the carb increases thru the narrow diameter of the carb throat (venturi), pressure decreases/vacuum increases. The higher the air velocity (higher engine rpms) thru the venturi, the greater the vacuum. These early distributors used this varying venturi vacuum level similar to the rpm-based mechanical advance to increase the timing advance as engine rpms increased. There was also a connection to intake vacuum to vary the vacuum based on engine load. These distributors must be matched to carbs that produce the correct low level vacuum signal from the venturi. When incorrectly matched with later carbs that produce higher vacuum signals for the SVDA/DVDA distributors... they were at full advance 100% of the time.

The method of setting ignition timing by revving the engine until the distributor advance stops increasing and then setting the advance in the 28-32BTDC range... only works for mechanical advance distributors. This is why you disable the vacuum advance when doing this with an SVDA/DVDA distributor. You are trying to make sure the mechanical advance (rpm-based advance) never takes you above a safe upper limit of 28-32BTDC total advance (initial + mechanical). This method does NOT include vacuum advance.
If you leave the SVDA/DVDA vacuum advance connected while you do this you get an unpredictable amount of vacuum advance added to your mechanical advance. If you assume the vacuum advance is adding 8deg or 12deg of advance you are risking over advancing the ignition timing.

Mon Nov 03, 2025 2:23 pm

DanCarr wrote: glutamodo wrote: The generation of spark does not matter if you're using a timing light and setting total advance. You can do retrofit electronic ignition into pretty much any VW distributor.

If it's an SVDA or DVDA distributor, to check total advance you only do the centrifugal, so with advance hose removed. If it's SVA (vacuum-only) the you HAVE to have the vacuum hose connected.

Thanks Andy. I am not sure which I have. This is a new engine. I will call the builder and find out. :)

Just called the builder. He confirmed this is a SVDA distributor and as you mentioned to remove the tube and plug the carb vacuum. Thanks again Andy! :)

Mon Nov 03, 2025 7:07 pm

I would include the 36 hp Beetle distributors in Ashman’s review. They were SVDA distributors. They were factory installed on Beetles for seven years so are fairly common. Here is a pictorial reference that can be useful in determining what type of advance can be found on a given distributor.

Note how tall the housing is. That indicates there is room in the interior for both a vacuum canister pull arm and mechanical advance weights.

Next is an SVDA from the late 60’s through the 1970’s. The housings are aluminum but they still are tall enough to accommodate both vacuum and mechanical advance.

Here we have short body distributors, early of cast iron and 1960’s of aluminum. There isn’t enough room to accommodate mechanical advance weights. These are vacuum advance only.

These are a bit taller but the absence of a vacuum canister indicates that they are mechanical advance only.

Lastly we have DVDA distributors. These did not come into production until the 1970’s as a smog limiting design. The body is tall enough for both mechanical and vacuum advance. The vacuum advance/retard has two vacuum ports, one on each side of the canister.

Tue Nov 04, 2025 5:37 am

[quote="ashman40"] DanCarr wrote: hy wouldn't you checkld check/adjust your timing AS YOU ARE DRIVING. This is so you can confirm the ignition timing with the engine under load (eg. climbing a steep hill) and while not under load (eg. cruising at constant freeway speeds). Obviously, this is not practical.

Absolutely. Early automobiles had the ability to do this from the dashboard or steering wheel. This may have contributed to premature engine failure due to drivers leaving it too retarded or advanced. The feature was eventually dropped.