Engine missfires only between 1400 to 2000

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kansas buggy
Posts: 142
Joined: Mon Aug 09, 2004 6:48 pm

Engine missfires only between 1400 to 2000

Post by kansas buggy »

Having trouble tuning a 2.0L that has a hyd cam. It has dual 44IDF (new) they have 32 vents, 200/55/130. The ignition is mallory Unilite Vac verison. Vac currently not connected. Timing is 10deg inital 27 total mechanical . Dizz has the gray springs. Wires plugs and mallory parts are all new. Location is Kansas City (600 ft atl.)

Here is a description of the miss fire. Idles fine no miss fire untill around 1400 then it miss fires. But not on a given cylinder. The miss fire goes away before 2000 then it smooths out. Appears to be fuel related. New fuel from the gas station. Fuel pressure 3.25psi. Have removed and cleaned both carbs. No improvment. Yes there is a filter on the fuel line before the pump.

Runs strong above 2000 for a hyd cam engine.

Any thoughts?
MASSIVE TYPE IV
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Joined: Fri Jul 07, 2000 12:01 am

Post by MASSIVE TYPE IV »

You need to narrow it down to one cylinder..
Sounds like an idle jet to me.
kansas buggy
Posts: 142
Joined: Mon Aug 09, 2004 6:48 pm

Post by kansas buggy »

I agree it sounds like a idle jet. Thats why I dissambled both carbs and cleaned everything. Still have the same symptom.

I must be missing somthing!!!!!
MASSIVE TYPE IV
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Joined: Fri Jul 07, 2000 12:01 am

Post by MASSIVE TYPE IV »

intake leak??
Guest

Post by Guest »

Have sprayed around the maniflods. No leak detected.

As a point Carb's, manifolds, ignition are all new. So the miss has allways been there since I have been able to run the engine.

The enging is a reffuge that I will use to learn VW methods and then build a 2316.
MASSIVE TYPE IV
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Joined: Fri Jul 07, 2000 12:01 am

Post by MASSIVE TYPE IV »

do a compression check and a leak down check. You may have a tight or burned valve.

Pull plug wires while its running to get the issue down to 1 cylinder then you can treat it accordingly.
kansas buggy
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Joined: Mon Aug 09, 2004 6:48 pm

Post by kansas buggy »

I did have the valves ground so I dont think its a burnt valve. I will double check the adjustment to make sure they are not to tight.

Will also do the leak down test. But would be supprised if it shows anything.

If that does not turn up anything, then I will take it to someone more experianced to see what they can find.
kansas buggy
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Joined: Mon Aug 09, 2004 6:48 pm

Post by kansas buggy »

Thanks for the good help.
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raygreenwood
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Post by raygreenwood »

Actually...if I din't know he had a mallory....and didn't know he symmetrical springs (ie both the same)...i would think it sounded a bit like a sticky advance transition. Check things just the same. Ray
914fan.

Post by 914fan. »

Are the carbs syncronised? Try turning in the idle screws to see if each cylinder responds. I have had lots of intake leaks and have not had this problem with carbs. However, that is the exact problem I had with intake leaks on FI. It could be a leak. The leak could be on the underside of the intake at the head and difficult to spray. I have found that the coper rtv is helpfull with sealing at the head. Are the air bypass screws turned in all the way? If not are they turned out about the same amount? I would say probably no more than 1/4 to 1/2 turn difference between the most closed to the most open. (by the way the air bypass screws act as a built in vacume leak. They allow more air to bypass the idle jet therby leaning out that cylinder.) Are the idle speed screws turned in almost the exact same amount on each side with the carbs synced? If not then the one that is more turned in (butterflies more open) probably has a leak or open bypass. The settinge on the two carbs (realy 4) should all be very close to eachother. I dont forsee that any one cylinder or bank would have drastically different settings. You should be able to swap carbs side to side with little to no adjustment. At least for the non dyno tuned engines. Good luck and let us know how it turns out.

Lastly check the gasket at the carb to manifold. It is not likely to be the problem but you never know.
MASSIVE TYPE IV
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Post by MASSIVE TYPE IV »

Yep.. Also ensure that the carbs are BOTH progressing at the same rate. Sometimes the linkage must be adjusted on the hex bar to achieve equal progression off idle.

Its easy enough to check, just watch the throttle stops and their distance right off idle, they should be identical.
kansas buggy
Posts: 142
Joined: Mon Aug 09, 2004 6:48 pm

Post by kansas buggy »

This is a new set up...ie these componets have never been operated together and are new. So anything is possible.

I did check the advance mech when I changed the springs and there did not appear to be anything wrong.

I also checked the linkage adjustment. Both carbs hit the WFO stops at about the same time. So to me they are adjusted correctly. Am I right?

I did sync the carbs using snail. The engine does idle smooth.

These are the first Webers I have tried to tune so my knoledge is suspect. To me the miss is fuel related. What you all have been suggesting is where I feel the problem is. I will re-check the vack leaks and the ignition. If that does not do it then I need to locate someone who has experiance with the type 4 and carbs to determine what I am missing.

Again thanks for the help wth the miss.
914fan.

Post by 914fan. »

Nobody else has said it so I will

dont overlook your fuel pressure and your float settings. The floats *could* have bees set incorectly at the factory and / or your pump could be outputting to much or little preasure. If all else fails remove everything and start over. (45 min. tops)

Does the miss happen when you rev it in nuetral or when you are driving with a load on the engine or both. If only when driving then the leak could be small enough that it is not there without a load (and higher vacume)

Good catch Jake. Forgot to mention the linkage
kansas buggy
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Joined: Mon Aug 09, 2004 6:48 pm

Post by kansas buggy »

Fuel pressure is 3.25psi according to the Holly pressure guage.

The miss occurs both reving the engine without a load or while driving (loaded).

Again it appears to me to be related to the throttle position. Like when it changes circuits because you have uncovered a different port. In fact thats why I have been chassing the idle circuit. With the butterfly closed for idle its not missing. When you crack it enough to rev the engine a littel it picks up a miss. When you further open the butterfly the miss goes away.
MASSIVE TYPE IV
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Joined: Fri Jul 07, 2000 12:01 am

Post by MASSIVE TYPE IV »

Just because both carbs hit the stops at the same time means little. Most of the time when I see this happen it occurs in the lower progression circuits right around 1500-2K like you explain.

BOTH CARBS MUST OPEN AND PROGRESS AT EXACTLY THE SAME RATE.

We call the problem "carb leading" because one will lead the other and cause a stumble/miss at low revs.

Crack the carbs open and measure the gaps at 1/8 and 1/4 throttle. If they are not exacting thats the issue. You can move the linkage arms laft and right on the hex bar to adjust the progression and cure the issue if they are not perfect.

I'd bet 50 bucks its the problem.
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