302 Engine and Carb issues

[Chuck]

We need some advice and suggestions. Just now getting the engine going since the rest of the GT is done and we are having some issues.

<B><U>Engine specs:<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com

302 ci.
Compression 9.37 : 1
Chamber volume: 60 cc
Comp cam / hydraulic rollers
Dart Pro head
Intake valves: 1.94
Exhaust valves: 1.60

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Carb:
Weber IDF x 4. Venturi was changed from 36 to 32 by T and L while dyno testing to get appropriate performance. Idle jets are #60. The carbs are balanced per the air flow meter. Idle mixture screws out about 1 to 1 ¼ turn each. Idle stop screw about 1 turn past initial contact. We have tried the 'best lean' technique for setting the mixture screws, but it is really hard to tell whether it is correct or not.
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The engine starts right up and idles nicely around 800 to 900 RPM. When cold it spits a bit. Once warmed the spiting at idle is minimal. When the engine is driven or reved, however, it spits and backfires through the RPM range.
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The engine does not seem to have much power, but given the spitting and backfiring and vibrations it is hard to tell.
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The spitting does not seem to be from any particular cylinder. It is random.
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Now here is the real problem. There is a significant vibration, worst between 2500 and 3000 RPM. It occurs when the car is reved whether parked or while driven. The vibration is much worse than I would expect even for an engine bolted essentially directly to a monocoque chassis.<o></o>
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Some ideas:
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1. Timing? Perhaps it is naïve of me to assume that a dyno tested engine was timed correctly. (Not sure what the optimum timing would be for a 302).
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2. Balance? The engine is 28 ounce. Surely T and L did not use the wrong harmonic balancer / damper. But perhaps the flywheel from Kennedy Engineering was not 28 ounces? It had no markings on it or any written documentation when received but I was assured in a follow up phone call that it was 28 ounces.
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3. Valve issues? Rocker arms? The rocker arms that came with the engine have been known to have some issues, and I have a replacement set waiting ready to install.
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4. Carb adjustment? The car does not seem to have the dreaded ‘dead spot’ in the transition zone around 2500 – 3000, but given the vibrations we have not pushed it hard so can’t really say for sure. Can’t quite see how the carbs would be responsible for the vibrations.
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This is new territory for me and any suggestions on where we should start would be welcomed.

 

[bob peckham]

Chuck, sent you a pm

 

[Jac Mac]

I would suggest that your balance is incorrect from your comments- motors with the wrong flywheel or damper fitted show vibration around that RPM area after which it appears to diminish & then return again at higher RPM. If you hold onto either the front or rear of a rocker cover while the vibration occurs you might be able to discern which end of the crankshaft has the faulty component. Of course there are many other things that cause vibrations as well!!

 

[Randy V]

Chuck - Sorry to hear of your problems...

I feel that Jac is correct in that a balance problem with between the engine / flywheel (sounds like T&L did not test with your flywheel) / Balancer...

It's odd to see a 302 with 28oz imbalance. They are typically 50oz unless a stroker crank (331-347) was installed.

The camshaft you have - do you have the specs / part number? It will determine your firing order. If you have installed the wires to the old 302 firing order and the cam is the later design (351w firing order) that will give you fits and a lot of backfiring.

302 (Pre-82) 1-5-4-2-6-3-7-8
5.0 HO 1-3-7-2-6-5-4-8
351W 1-3-7-2-6-5-4-8

Another thing - If you have the firing order wrong, the engine will shake (vibrate) but it generally won't go away and then comeback as the RPMs increase..

 

[Mike Drew]

Here's a very stupid suggestion.

Check the firing order.

I had a friend who had similar issues with his motor; turns out he had two of his plug wires swapped. The exhaust masked the fact that two cylinders weren't firing; it ran like crap and backfired from time to time, and he didn't know why. It also shook like a wet dog at certain rpm, not others.

Remember that there are two different possible firing orders for 289/302/5.0 camshafts; if you installed a cam ground on the 5.0 pattern in an old block and assumed that the motor had the old firing order, that could be causing you all sorts of problems.

 

[Chuck]

Thanks for the suggestions. Here is some more info:

The firing order is: 1 3 7 2 6 5 4 8. We actually pulled all the wires at Run and Gun and tried both firing orders to make sure the firing order was right. With the other firing order it would not start.

The camshaft is described in the T and L spec sheet as follows: Comp, Part Number 3632 / 3613. (I could not find a match with those numbers). It is a mild cam. Then engine dyno at about 375 HP. Not a stroker, just a 302.

The flywheel came from Kennedy Engineering and was not balanced with the engine.

I am trying to confirm with T and L that it is in fact a 28 ounce balance (as I was told originally). I am also trying to confirm what damper was used to see if that might be an issue. Hope to get responses shortly. Are dampers typically marked with the balance weight? Getting to ours is a bit of a challenge since it is burried deep but if they are typically marked that would be a good place to start.

I started it up again last night after the neighbors went to bed and again noted that it started immediately and idled smoothly around 1000 RPM with minimal warm up. After reaching operating temperature it idled without significant popping or backfiring. Reved it up to 2500 - 3000 and it rattled the whole car. Reved it to 4000 RPM and it smoothed out but not as smooth as I would expect. It is still popping and backfiring between 1500 to 3000, but seems to pop and backfire a bit less at higher RPM. Could the carbs be an issue? Transtion circuit? I really don't intend to rev it any higher or any more until we get this resolved.

Chuck

 

[Bill Musarra]

Chuck,
Some of the dampners have add on weights and are neutral balance if not installed. It allows them to fit either engine. You will have a hard time telling without pulling the piece. It may have been left off by accident, or the wrong weight was put on. Just a thought. In any case you need to pull it to make sure. Your flywhell should have some holes drilled in one section it to allow for the "imbalance". Again, you will have to pull to know for sure. Kennedy sent me the wrong wheel with my order. Mine is internally balanced and they sent me one of the "unbalanced" wheels. It happens.

Bill

 

[chuck smith]

I would just have to think that if it were a balance problem it would not smooth out at higher RPM. If the wrong balancer is on, again I would think the engine would shake very badly at all RPM, although I suppose if the flywheel is very slightly out of balance or some other component is then you might get those symptoms as harmonics come into play. I vote for unbalanced carbs. With webers ,if one carb is not feeding well at certain RPMS, you will get vibration

 

[Tim]

I would plump for a balance issue. More to the point a couple imbalance. Heavy points opposite each other at opposite ends of the crank. This will run smoothly at some speeds but vibrate badly at certain speeds. Your crank should be balanced fully assembled with bob weights on the rod journals, flywheel and clutch and balancer on. The "Harmonic" balancer must match your set up. Your vibration at 2500-3000 will be the major harmonic and it will smooth out and come back at 5000-6000, it is unlikley you will notice the 1250 harmonic as it will not be large enough.

The other issue of spit back is often caused but the engine going lean. I have seen too many people set the carbs up with out doing some light throttle passes on the dyno. you may find your primary circuit is too lean on light throttle at higher revs.

 

[Jac Mac]

Your cam if a Comp Cams make is an 'xtreme energy hydraulic roller', has a #3632 profile on what I assume is the intake lobe. (273° rated, 224° @0.050" and 0.347" lobe lift )---#3613 on the exh (282° rated, 230° @ 0.050" and 0.320" lobe lift ). It sounds like a cam that T/L have ground to there own request as many engine builders do . The lobe seperation angle might be on your data.

 

[Doug S.]

Then engine dyno at about 375 HP. Not a stroker, just a 302.
Chuck

Just curious, how did it run on the dyno and was it a bench dyno or was that 375 HP figure from a chassis dyno? If it ran OK on a bench dyno, could the problem be in the MSD unit or perhaps a less than secure ground wire that is "excited" into poor contact at a particular RPM range??? If it was a chassis dyno, well, then what was changed between the dyno pulls and now?

Doug

 

[Chuck]

Good suggestions. Before we take apart the tranny or buy a new MSD box thought we would explore other issues this afternoon. Like the carbs. Here is what we found.

The main venturis are 32 mm. Per the spec sheet the first pass was with the 36 mm venturis.

The idle jet is 60. The air jet is 175. The main jet is 135.

We have the idle pretty well set. Once warmed up it does not pop and back fire while idling.

Around 2500 - 3000 it gets nasty. Lots of backfiring and a bit less popping, but still way too much. Backfires less above 4000.

We replaced the air jets. Our reasoning was they had more to do with the transition circuit in the 2500 RPM range where we are having all the issues. First went up to 180. It seemed to run better, a bit less poppoing and backfiring, but still too much. For a point of reference we then went the other way, to 170. It was worse, so we went back to the original setting of 175.

Before we do any more experimenting with the air and main jets, does anyone have any suggestions? We may try 185 on the air jets, since 180 was better than 175.

The engine was bench dyno'd. Relevant changes since dyno testing would include the ignition and the flywheel. the dyno print out starts at 3500 RPM, so the dyno range is above the point where the nasty vibs occur.

I am going to check for any loose wires on the ignition system. We changed out the coil last night and that made no difference. I would prefer to hold off buying a new MSD box until we are sure that is the issue.

Keep in mind that the adaptor plate between the engine and tranny is bolted directly to the chassis - there are no bushings whatsoever. So engine vibs will obvioulsy be transmitted into the rigid chassis. But to my inexperienced eye, it should not feel like a "magic fingers" mattress set in the supper high mode at the local Hotel Six when revving at 2700 RPM.

 

[Terry Oxandale]

I've got a stroker 351W (383) solidly mounted to a chassis and vibration is not an issue at any RPM (it was balanced though). If indeed this is shaking the whole car, then to transfer that kind of force to make the more massive chassis vibrate as described indicates to me something other than fuel or ignition is the problem (e.g. balancing), ESPECIALLY with 375 HP on the dyno. For your set-up (which appears to be fairly conservative), this power level appears to be about right, which again indicates to me that it's not fuel or ignition.

I ran a poorly built motor previously that fowled plugs to the point where at times I would be running on 6 to 7 cylinders if I didn't keep the plugs cleaned, and I could feel the added vibrations from the misfire, but it was nothing compared to your description.

 

[Frank]

Since your idle screws are 1 1/4 out or so. Try cranking them all in a 1/2 a turn or more. See if that changes the vibration around.

Did any of the carbs need more or less ( Big difference ) air than the others.

What distributor do you have. MSD pickups get filthy. Whats your initial and total timing set at.

Flywheel dowels installed for the clutch/pressure plate?

Pull one plug wire at a time and see if theres any change in that RPM range.

Is the vibration noticeable on quick revs or just holding it at the bad RPM range.

Sorry for the rambling, its late

 

[Mike Drew]

Slap a simple intake and a Holley on it and see if the problem persists? That will tell you instantly if it's a tuning issue or a mechanical balance issue....

 

[Chuck]

Good ideas and suggestions. Thanks.

Mike: I have been considering swapping out the Webers for a simple Holley. May be looking for some suggestions. Perhaps something in the 500 to 600 cfm range?

Frank: Here are answers to the questions:

1. Cylinder number one required that the mixture screw be out an extra half turn or so. The exhaust manifold was cold until we backed it out a bit more.

2. Small MSD distributor. We pulled it and it looked ok. It is new, as is the entire engine build.

3. Pics of the flywheel and clutch are attached.

4. The vibration is present at 2500 - 3000 regardless of whether one is revving the engine or holding the speed constant.

5. Will try turning in the mixture screws a half turn each. My guess is it will just die and not idle. Will need to turn in the idle stop screw a bit, I am guessing, to keep it running.

6. Will try pulling plugs again.

7. Have not checked timing since the engine was dyno'd. That requires removing the fire wall and some gyrations, but is next on the list.

Let me know what you think. Thanks.

 

[Ian Anderson]

Does your distributor use a vacuum to get the advance right?

If so is it connected to a single carb?
If so I've heard these cause pulsing and move the timing erratically.
If no vacuum then disregard.

What about advance springs inside the distributor?
Again I'm not sure what you've got inside it

I know on Rover engines there has been a spate of failed rotor arms - cheap China units that come in original looking boxes but fail after only a few hours running - coould it be the same with your rotor - cheap & easy to replace

Ian

 

[Chuck]

Ian:

The distributor advance is mechanical, not vacuum. I have not made any changes to it.

 

[Pat Buckley]

I think you have a flywheel or damper that is incorrect.

 

[Jac Mac]

I think you have a flywheel or damper that is incorrect.

Yep, & if it is the longer he keeps on playing with carbs etc the less he will have to do to fix it as it will start to disassemble itself!