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Santaji
Samba Member
Joined: December 29, 2007
Posts: 105
Location: Mumbai, India
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 Posted: Fri Dec 27, 2019 11:15 am Post subject: HPMX/IDF carbs jetting, how to get beter cold starts |
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The Bus caught fire last December. We were able it get it back on the road, but unfortunately the fuel-injection was toast. We are now running dual EMPI 40HPMX carbs.
Performance is pretty good once it warms up, but its very difficult to start, and there is some backfiring at low RPMs, especially when the engine is cold.
The jetting is as follows:
Idle jet - 52
Main jet - 130
Air correction jet - 170
Emulsion tube - f11
Venturi is 28mm
We tried going larger with the main jet and air-correction jets, but she would run too rich and was almost un-driveable....
Also tried 55 idle jets, which improved cold starts and got rid of the backfire problem, but the overall performance was much worse. (seemed too rich)
Looking for suggestions on improving the cold starts and running. If anyone is running similar carbs on a 2.0L Type-4, please mention what size jets you are using...
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'78 Westfalia '63 RHD Beetle '62 Fiat 1100 |
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SGKent 
Samba Member
Joined: October 30, 2007
Posts: 41031
Location: Citrus Heights CA (Near Sacramento)
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| Posted: Fri Dec 27, 2019 2:53 pm Post subject: Re: HPMX/IDF carbs jetting, how to get beter cold starts |
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you won't solve cold weather starting problems by re-jetting. You need a choke on each carb and preheat while it warms up, or learn how to start it without one, then warm the engine up some before you drive away.
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Santaji
Samba Member
Joined: December 29, 2007
Posts: 105
Location: Mumbai, India
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| Posted: Fri Dec 27, 2019 3:08 pm Post subject: Re: HPMX/IDF carbs jetting, how to get beter cold starts |
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| SGKent wrote: |
| you won't solve cold weather starting problems by re-jetting. You need a choke on each carb and preheat, or learn how to start it without one, then warm the engine up some before you drive away. |
The weather doesn't get very cold here (never need the choke on my Beetle or Fiat). I can get the Bus running in 2 or 3 attempts by pumping the pedal then keeping the throttle slightly open, but idle is very rough until engine warms up.
There's an occasional backfire at low RPMs too, even after the engine warms up, hence I'm suspecting the jetting.
Like I said, 55 idle jets would "solve" the starting and and backfires, but other then idle did not run very well, so I went back to the 52s.
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skills@eurocarsplus
Samba Peckerhead
Joined: January 01, 2007
Posts: 16864
Location: sticksville, ct.
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| Posted: Fri Dec 27, 2019 3:10 pm Post subject: Re: HPMX/IDF carbs jetting, how to get beter cold starts |
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| SGKent wrote: |
| you won't solve cold weather starting problems by re-jetting. You need a choke on each carb and preheat while it warms up, or learn how to start it without one, then warm the engine up some before you drive away. |
^^^^this
your jetting sounded good to me. it's a learn to live with it kind off deal, or learn what the carbs want when it's cold. my dell 36's want 10 pumps stone cold before I even get a fart. 2 more after the fart then feather the throttle for a couple mins till it holds an idle
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busdaddy
Samba Member
Joined: February 12, 2004
Posts: 51130
Location: Surrey B.C. Canada, but thinking of Ukraine
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| Posted: Fri Dec 27, 2019 4:12 pm Post subject: Re: HPMX/IDF carbs jetting, how to get beter cold starts |
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As already mentioned it sounds like it's jetted fine already, you'll have to work on your cold start and warm up procedure unless you want it to run crappy when it warms up.
There are chokes (actually cold start enrichment valves) that attach where that blanking plate on the other side from the accel pump is, I'm not sure where you would buy them though, or if the Chinese carbs even have the same drillings under that plate as the Italian ones. Then you'll also have to rig up a long cable system to operate them and figure out how not to forget they are on once it's warm.
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SGKent
Samba Member
Joined: October 30, 2007
Posts: 41031
Location: Citrus Heights CA (Near Sacramento)
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| Posted: Sat Dec 28, 2019 12:39 am Post subject: Re: HPMX/IDF carbs jetting, how to get beter cold starts |
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| Quote: |
| There's an occasional backfire at low RPMs too, even after the engine warms up, |
| Quote: |
| Performance is pretty good once it warms up, |
| Quote: |
| We tried going larger with the main jet and air-correction jets, but she would run too rich and was almost un-driveable.... Also tried 55 idle jets, which improved cold starts and got rid of the backfire problem, but the overall performance was much worse. (seemed too rich) |
Let's get your story straight without the twisting.... changing stories won't fix your problem either. You need to jet it how it runs best when warm. You need a thermostat on the engine that works too. And then you need to learn how to start it and warm it up in winter. Starting the engine at 45F is different than starting it at 70F. Fuel doesn't vaporize as well at 45F as it does at 70F.
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Wildthings
Samba Member
Joined: March 13, 2005
Posts: 50338
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| Posted: Sat Dec 28, 2019 12:55 am Post subject: Re: HPMX/IDF carbs jetting, how to get beter cold starts |
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| What about going with the 55 idle jet and dropping the main jet to 125, leaving the air correction jet alone. |
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Santaji
Samba Member
Joined: December 29, 2007
Posts: 105
Location: Mumbai, India
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| Posted: Sat Dec 28, 2019 1:08 am Post subject: Re: HPMX/IDF carbs jetting, how to get beter cold starts |
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| SGKent wrote: |
Let's get your story straight without the twisting.... changing stories won't fix your problem either. You need to jet it how it runs best when warm. You need a thermostat on the engine that works too. And then you need to learn how to start it and warm it up in winter. Starting the engine at 45F is different than starting it at 70F. Fuel doesn't vaporize as well at 45F as it does at 70F. |
Sorry for the confusion, I meant that once it warms up, it does backfire occassionally at slow speed, but other then that it's smooth and has decent power.
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SGKent
Samba Member
Joined: October 30, 2007
Posts: 41031
Location: Citrus Heights CA (Near Sacramento)
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| Posted: Sat Dec 28, 2019 10:48 am Post subject: Re: HPMX/IDF carbs jetting, how to get beter cold starts |
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there are many reasons an engine backfires. Running lean can cause it (carb backfire), or being too rich combined with a little air leak into the exhaust (exhaust backfire). Ignition timing or leaky plug wires, carbon tracking in the cap can cause it too if it spits back into the carb. As I said, you jet the engine when it is warm and then you learn how to start it when cold with those settings. You don't jet it to start cold. Generally you choose a main air jet for fine tuning the top and bottom end - 3 air correction jets usually equal one main size. You use an idle air jet that sets the air to fuel mixture screw about 2 1/2 to 3 turns open.
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