Hmm... slight engine miss... any suggestions? (see or rather hear video)

[Jeff K]

Ok, we got everything back together. Started up just fine!
The engine has Stage 2 cams and exhaust.

Engine was dissembled, hot tanked. New bearings/rings and a valve job.

I adjust the fuel mixture with a dwell meter. I readjusted the valves (cold after a short run) 7 thousandths. New plugs properly gapped. I set the timing.

I’m hearing a slight miss. On the video it sounds more metallic, but it sounds like lugging sounds. Like a two stroke motor set too rich. (From my model airplane days)

https://www.youtube.com/watch?v=BUmwHSM4pts

I’m thinking it might be spark related.. but it’s inconsistent. The fuel system operated perfectly before the rebuild of the engine. Only different is the stainless lines in the fuel system and fuel filter.

The engine starts perfectly when cold, warm and hot.

Any suggestions?
Thanks
Jeff Klavir

Ok, these items are brand new. (not a complete list)
Fuel Filter
Stainless fuel lines
Rotor
Plugs
Engine - Rings, bearings, chains, tensioners, chain guides and valve job.


Less than 10K Miles
All hoses
Distributor (electronic)
Plug Wires
Fuel Distributor rebuilt.
Fuel Pump
Radiator
Water Pump
Injectors (from DMCCA)

[NightFlyer]

Is the vacuum advance for the distributor working properly? Check the solenoid and make sure that the electrical and vacuum lines are hooked up properly. Then check to see if you're actually pulling vacuum at the distributor - it should only be pulling when the engine temp is greater than 104F and never when the engine is at idle. If it's pulling vacuum constantly or fails to pull when the engine is above 104F, when you either have a bad or improperly calibrated switch ( idle microswitch or thermal vacuum switch) or a bad solenoid.

Absent that, only other thing I can think of to have you check for is vacuum leaks.

This assumes that the fuel system is functioning properly.

Best luck!

[Jeff K]

Thanks for the reply!

Ok, It seems to miss more when the engine is warmed up.

I removed the vacuum line to the distributor during idle, no change (See pic 1)
I removed it when it running over idle (about 3k rpm) - definite change
2688226883

I can assume that part is working? Not really sure if that's a good test.

See pic 2. Could I have missed a tooth for the distributor timing? Can I be one tooth off? I had the engine at TDC, by the book. (Insert rod in through plug at the top.)

Jeff

[NightFlyer]

Thanks for the reply!

Ok, It seems to miss more when the engine is warmed up.

I removed the vacuum line to the distributor during idle, no change (See pic 1)
I removed it when it running over idle (about 3k rpm) - definite change
2688226883

I can assume that part is working? Not really sure if that's a good test.

See pic 2. Could I have missed a tooth for the distributor timing? Can I be one tooth off? I had the engine at TDC, by the book. (Insert rod in through plug at the top.)

Jeff

Sounds like the advance is working properly.

Yeah, I'm thinking that the timing is off. And yeah, you could be a tooth off. But before you check that, check for any other vacuum leaks.

You can do this by spraying carb cleaner around the engine while it's idling. If there's a sudden increase in RPM when you're spraying the carb cleaner, then you know you have a vacuum leak and roughly where to look for it. Areas to pay particular attention to are where the idle air motor enters into the lower housing of the mixture control unit (aka 'the pipe of agony') and around the throttle body.

If you don't have any detectable vacuum leaks, then I'm all for resetting the timing.

[Bitsyncmaster]

It would not matter if you were off a tooth as long as your were able to set the timing with a timing light.

If you have not set with a timing light that should be the first thing you do after installing a distributor.

[Jeff K]

It would not matter if you were off a tooth as long as your were able to set the timing with a timing light.

If you have not set with a timing light that should be the first thing you do after installing a distributor.

I set the timing with a timing light Dave, all good there!

[hmcelraft]

Does the misfire pattern change with the engine under load? ie does it become steady? "pop" in the exhaust or intake?

Is it one cylinder or various/" floating". It sort of sounds like an intake "pop" in which case I would say a cam is out of time - But really hard to say with just this info. Were the heads cut down any?

[Jeff K]

I can't hear it under load, definitely hear it deaccelerating.
It's not consistent, but seems to increase with rpm in neutral.

The heads were not cut down. Maybe the builder missed a tooth when timing it?

When I adjusted the valves, it seemed correct, but i probably couldn't eyeball one tooth.

Can't say if it's one cylinder or not. Can't tell.

Does this help?

[Silverbullet]

Who was the builder?.... What do they say?

Craig

[hmcelraft]

Just random thoughts - but, you might try some carb spray on the intake manifold seals. You may have one or more out of its seat and leaking. A smoke test would show any leaks too. Does the engine miss under load or anything you notice at least?

[Jeff K]

Just random thoughts - but, you might try some carb spray on the intake manifold seals. You may have one or more out of its seat and leaking. A smoke test would show any leaks too. Does the engine miss under load or anything you notice at least?

I'll try that.

It seem fine under load when cruising or accelerating.

When de-accelerating, it sounds bad.. like popping and lugging.

We took the entire drive train and reinstalled it. Double checked everything, all new o rings too. But who knows, worth a try!

I wonder if any place rents the smoke type leak tester?
Jeff

[David T]

I'll try that.

It seem fine under load when cruising or accelerating.

When de-accelerating, it sounds bad.. like popping and lugging.

We took the entire drive train and reinstalled it. Double checked everything, all new o rings too. But who knows, worth a try!

I wonder if any place rents the smoke type leak tester?
Jeff

Check:
valve adjustments
fuel injectors
spark plugs
vacuum leaks
reluctor gaps to the impulse coil inside the distributor.

[Jeff K]

Check:
valve adjustments
fuel injectors
spark plugs
vacuum leaks
reluctor gaps to the impulse coil inside the distributor.

Thanks for list David.

I changed the plugs to brand new bosh silver and gapped them.
I checked the valve clearance twice, (7 thousandths cold)
Vacuum leaks, I'm going to take it to a buddy that has a smoke unit, have not done that yet.
Injectors have about 8k, bought new from DMCCA. I do have a spare set though.

I have no idea what reluctor gaps to the impulse coil inside the distributor means.. can you please explain that.

It's so weird. It starts and idles perfect. It accelerates fine, cruises fine. de-accelerating and under no load I hear it miss.

I took it to the engine build. He seems to think Vacuum leak or something with the ignition. Said valves sound fine.

Much appreciated
Jeff

[Ron]

You might get a few clues pulling the plug wires one at a time and note differences in the miss etc. Look for something like, pulling one will create a constant dead miss and the miss in question goes away, while pulling the others creates a dead miss and the one in question remains (the one points to ITS plug, fuel, ignition, vacuum, etc.), for example. Or, anything showing one different than others....

I can't tell from the vid, but is something loose in the exhaust system out of the question? (I couldn't detect the engine shake etc when it was missing...could easily be me tho ;-)

[NightFlyer]

It's so weird. It starts and idles perfect. It accelerates fine, cruises fine. de-accelerating and under no load I hear it miss.

I took it to the engine build. He seems to think Vacuum leak or something with the ignition. Said valves sound fine.

Much appreciated
Jeff

Wait, so it only misses when de-accelerating?

Could be bad/seized bypass valve springs on the throttle plate.

[David T]

Thanks for list David.

I changed the plugs to brand new bosh silver and gapped them.
I checked the valve clearance twice, (7 thousandths cold)
Vacuum leaks, I'm going to take it to a buddy that has a smoke unit, have not done that yet.
Injectors have about 8k, bought new from DMCCA. I do have a spare set though.

I have no idea what reluctor gaps to the impulse coil inside the distributor means.. can you please explain that.

It's so weird. It starts and idles perfect. It accelerates fine, cruises fine. de-accelerating and under no load I hear it miss.

I took it to the engine build. He seems to think Vacuum leak or something with the ignition. Said valves sound fine.

Much appreciated
Jeff

Get out your Workshop Manual and go to C:05:02. Intake is .004" and exhaust is .010" cold. The reluctor gaps are inside the distributor. It seems that if the impulse coil was ever replaced whoever does it is not always careful to make sure they did not disturb the gapping of the reluctor (8) to the impulse coil (17). If ALL of the gaps are not equal the idle can never be smooth since the coil is not firing equally to all of the cylinders. Refer to M:01:01 Fig. 2 and M:01:04 Trigger/Stator air gap. You are supposed to use a brass feeler gauge. Make SURE all of your plugs are gapped as close to each other as possible. It is also possible you have a bad spark plug. Rare but not unusual. Make sure they are all torqued in to spec and every wire is snapped on. You MUST hear and/or feel the boot snap when you connect it to the plug. While you are checking things do a compression test. Maybe one cylinder is low?

[Ron]

Get out your Workshop Manual and go to C:05:02. Intake is .004" and exhaust is .010" cold.Doesn't the stage 2 have different cams??

[Bitsyncmaster]

Wait, so it only misses when de-accelerating?

Could be bad/seized bypass valve springs on the throttle plate.

+1
I was thinking the same thing. One day I want to try my old throttle body (bad springs) with the deceleration springs soldered shut. I'm wondering if my new throttle body springs are going bad.

[Ron]

Hmmm...seems that the system would try to compensate (maybe throwing the idle off a bit...easy to check anyway...), but it seems that any miss would be consistant and even , ie not random/intermittent ??

[David T]

Doesn't the stage 2 have different cams??

It does but since the valve adjustment is mainly a function of thermal expansion of the valves, it should be either the same spec or close to it. Typically the exhaust is always a bit looser than the intake because it gets hotter and will expand more. If you are that concerned about valve specs (and you should be) place a call to DMCH to confirm what it should be for the Stage II. I would bet it is not .007 for I and E.

[Jeff K]

It does but since the valve adjustment is mainly a function of thermal expansion of the valves, it should be either the same spec or close to it. Typically the exhaust is always a bit looser than the intake because it gets hotter and will expand more. If you are that concerned about valve specs (and you should be) place a call to DMCH to confirm what it should be for the Stage II. I would bet it is not .007 for I and E.

I have DPI cams, he said 7 thousands for both.. I'll double check that.

Hey David, how do you actually measure .25 on the reluctor gaps? Where exactly is that? Is there a photo somewhere?

Also, how do you rotate the engine to check the valves without taking off the exhaust and using a breaker back with a 36mm?

I read someone put it in 5th and rotated a wheel while blocking the other.. I think..

Thanks
Jeff

[SS Spoiler]

Instead of putting it in 5th gear, wouldn't first gear give you more "leverage?'

[Ron]

It does but since the valve adjustment is mainly a function of thermal expansion of the valves, it should be either the same spec or close to it. Typically the exhaust is always a bit looser than the intake because it gets hotter and will expand more. If you are that concerned about valve specs (and you should be) place a call to DMCH to confirm what it should be for the Stage II. I would bet it is not .007 for I and E.I'm with you, but from experience, Id give it no more than a 50-50 chance of being the same as OEM since it is not a "replacement" grind (and not really "hydraulic", as we old schoolers know them ;-).

EDIT: Now you guys have me confused...DMCH v/s DPI specks???