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Engine troubleshooting help 1971 411
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jensvagelpohl Premium Member
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PostPosted: Wed Aug 14, 2019 6:18 am    Post subject: Engine troubleshooting help 1971 411 Reply with quote

I'm in the process of buying a low-mileage all original 1971 411 and the owner (who lives a 2.5 hour plane ride away) is trying to get it to run well. The FI and ignition components are all original, this is the "W" engine mated to an automatic transmission.

Symptoms:
- starts up pretty willingly, but idle too low and stumbling
- constant backfiring into the intake when I push down on the accellerator
- no power when driving, I think I got it up to 50
- backfiring doesn’t change much as the engine gets warmer
- keeping the accellerator in one position, inclucing all the way down, causes more backfiring than actual speed increase
- the only way to get accelleration for split seconds is to “pump” the acellerator, by backing up and pushing down carefully, at that moment the engine runs well and accellerates for a sub-second period, and then returns to the no-power situation

He says all basics have been gone through:
- distributor and trigger points cleaned up
- dwell set correctly
- timing [email protected] RPM as per the literature
- he has checked nearly all electrical components with the help of the Harold Glenn guide and they check out electrically

Yet the symptoms persist. The car ran much better originally he says, so it's something that changed within the last year.

This weekend he will check if the MPS holds a vacuum, and he wanted to see if there are any ground straps that are loose.

Does anyone have more ideas? There must be something basic wrong, seeing how all RPM ranges are affected.
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KTPhil Premium Member
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PostPosted: Wed Aug 14, 2019 7:48 am    Post subject: Re: Engine troubleshooting help 1971 411 Reply with quote

This is D-jet, right? For starters, I'd start with timing and vacuum leaks. Timing includes idle setting, proper vacuum can operation, and check the rotor springs back when twisted. Also check the injector tip seals... a leak there lets air in that the FI won't know about, unlike other hose leaks the MPS will catch. Also check the MPS isn't leaking vacuum.
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jensvagelpohl Premium Member
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PostPosted: Wed Aug 14, 2019 8:05 am    Post subject: Re: Engine troubleshooting help 1971 411 Reply with quote

KTPhil wrote:
This is D-jet, right? For starters, I'd start with timing and vacuum leaks. Timing includes idle setting, proper vacuum can operation, and check the rotor springs back when twisted. Also check the injector tip seals... a leak there lets air in that the FI won't know about, unlike other hose leaks the MPS will catch. Also check the MPS isn't leaking vacuum.


Yes, it's the original D-Jetronic.

The ignition on that engine is not set at idle, but at 3500 RPM, and that has already been done. I will ask him to see if the mechanical advance springs back.

He has carefully sprayed some starter fluid around the injector bases with no change in idle, which most likely eliminates seal issues there.

This weekend he will test the mechanical function of the MPS, i.e. "does it hold vacuum".
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KTPhil Premium Member
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PostPosted: Wed Aug 14, 2019 9:57 am    Post subject: Re: Engine troubleshooting help 1971 411 Reply with quote

I'm not up on ignition timing on the T4s, but the only time I've heard to set timing at high rpm is with the 009-type mechanical distributors, which won't work with D-jet. Usually these engines are set at idle while paying attention to which hoses are connected/removed/blocked off. I can imagine if the advance was not working correctly, you could in fact set timing at 3500 rpm and then have it be incorrect for all other load/rpm conditions, causing your poor running. So I'd start by getting the timing correct, not just the setting, but verifying the mechanical and vacuum advance portions are functioning at 100%.

Next I'd check that the PCV valve and its related hoses are also correct. Unlike the Type 3, the Type 4 (at least the later ones) has a true PCV system and it can cause all sorts of poor-running if not 100% correct. It might help to get a photo of your breather/PCV setup including where any hoses lead, just to make sure they are connected correctly. I perused a few threads on the Bus forum and see there are many variations, which if crossed up, can really mess with its running. So let's be sure you have the correct setup for your model/engine, and that it's connected and functioning correctly. There are several good hose diagrams floating around here, but with the T2/T4 variations, I'm just not up to speed on what yours needs to be. Someone else surely can help us here!

Then as you say, have the MPS checked that it holds vacuum, and has the right electrical readings (no shorts, correct resistance).

It's not really worth checking other things until these are known good, since maybe 80% or more chance it's one of these.

Keep us posted!
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raygreenwood
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PostPosted: Wed Aug 14, 2019 11:55 am    Post subject: Re: Engine troubleshooting help 1971 411 Reply with quote

Timing with type 4 and D-jet is as listed....27 BTDC at 3500 rpm.

You have vacuum leaks....period.

If it ran better before you started working on it.....and neither of you are practised with D-jet.....you have vacuum leaks. You just dont know where to look. If you have not replaced or clamped 100% of your hoses.....yourself......in the past 5 years.....you hsve leaks.

Also...backfiring in the manifold. For that to actually happen.....one of three things must take place:

1. too much valve overlap ....and the only way to get that with atock parts is the distributor being out of time. So be sure you sre timing the engine on #1....and be sure the distributor is not out,180°.

2. Leaking valves from a bent or burned valve head. Do a compression test and verify.

3. Very poor or incorrect valve lash adjustment. D-jet is VERY senaitive to valve adjustment

Also ....90% of the time people think one of these engines is backfiring into the intake manifold....its not. Its backfiring in the exhaust.....usually due to leaks at the heat exchanger gaskets.

Also mistaken for backfiring.....is poor connectivity ....injectors not working or power being intermittent. If its the stock harness and you have been working on things and have not taken great, specific pains to work on the connectors.....not the plugs....the connectors inside of them.....then you automatically have issues.

You likely have ignition issues as well. Be sure.....its 350p rpm with hoses off.

Lastly....check that the PCV valve is working and that the MPS is holding vacuum and get back to us.
Ray
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KTPhil Premium Member
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PostPosted: Wed Aug 14, 2019 3:10 pm    Post subject: Re: Engine troubleshooting help 1971 411 Reply with quote

FWIW, the Muir book does list the W engine as being timed at 3500rpm (to 27° with vacuum hoses off) as you describe, but it also lists 5° statically, which may be a better baseline given the unknown advance status, and follow up with a check at 3500rpm to check for proper advance.
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raygreenwood
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PostPosted: Wed Aug 14, 2019 4:45 pm    Post subject: Re: Engine troubleshooting help 1971 411 Reply with quote

KTPhil wrote:
FWIW, the Muir book does list the W engine as being timed at 3500rpm (to 27° with vacuum hoses off) as you describe, but it also lists 5° statically, which may be a better baseline given the unknown advance status, and follow up with a check at 3500rpm to check for proper advance.


They both are pretty close actually. 27 BTDC at 3500.....is about.....on most of these engines...a range of 5 to 7 BTDC static. I say pretty close because really....these engines like a couple more degrees of idle timing. It seems to be that getting about 2° more static or at idle....only seems to equal about another 1° of dynamic timing reading at 3500. Some float in there somewhere.

This type of misfire.....can actually BE a misfire.....if you have carbon tracking in the cap, heavily carbonized rotor or cap or if the distributor is really worn and sloppy. I have seen it more than a few times. One or more cylindefs can get an arc when they are not in the correct spot so you get ignition when the valve is partially open or at some other place in the stroke.

Check and adjust valves first......then carefully check that the distributor is properly clamped and also check for slop in the points plate. It has a ton from the factory that I take out when I do a plate refurbishment.

Also...sometimes it can feel like the system is misfiring into the intake....and has zerious loss of power....when the injection system is actually cutting out. Literally cutting off and on rapidly.

The three items that commonly cause this are poor connections at the trigger plate plug, loose ground wires on the case centerline under the manifold and the main power relay griund to the chassis loose. Ray
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Lars S
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PostPosted: Fri Aug 16, 2019 7:06 am    Post subject: Re: Engine troubleshooting help 1971 411 Reply with quote

Backfiring into the intake could possibly be because if running lean. If the engine has been sitting for a longer period it is not unusual that one or more of the injector needles get stuck/partially stuck or the injector is clogged/corroded.
Sometimes they will get free/clean after a bit of running sometimes not.


/Lars S
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