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Old 08-31-2010, 11:11 AM
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Backfire through Carb?

Finally got my custom build 302 up and running but can't get rid of an idle backfire and backfires on initial acceleration. Have adjusted Holly accelerator pump, idle jet screws, increased accelerator squitter from 31 to 37, squitter engages immediately. Increased power valve from 6 to 9.5 (vacuum = 11 at idle) Adjusted floats. Moved timing all over,(seems to like it around 3 to 4 degrees, but it still backfires. Runs good at higher rpms, no backfire at all. Compression test shows cylinders from 175 to 183. I am starting to suspect crossfire in the distributor, but it's all new. Need help?

Engine is as follows:

91 Ford Roller cam 302 block.
KB flat top pistons (about 10 to 1 compression).
AFR heads 185's with 58cc. (1388's)
Summit Racing stage II vented intake manifold.
Holly 4150 Street Avenger with vacuum advance secondary’s, electric choke.
Holly Blue fuel pump set at 7 psi.
Comp Cams XE264HR-14, roller cam, 110 degrees separation (not too radical) set at about 3 degree advanced.
Mallory SS42 distributor with vacuum advance.
Mallory Hyfire 6AL
Mallory coil 29440
Taylor 409 - 10.4mm plug wires
Champion RC9YC (one step colder than standard).
Moroso electric water pump.
Full length ceramic coated headers, 1 5/8" to 3" H pipe, and dual 2 1/2" Flowmaster Delta 50's.
No smog equipment, egr, or PVC on engine (will be going in a 64 Falcon).

I refitted plug wires at dist. cap. No change. Plugs: # 1,2,& 6, are black with soot. The others are a bit darker than before, but that's likely because of the richer carb settings. Ran compression check: 175,175,178,183,180,180,180,180,180. Could it be that the intake manifold isn't seating well with the heads? But wouldn't that show on the vacuum readings?

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Old 08-31-2010, 03:49 PM
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You have to use a diagnostic vacuum gage w/an un-damped needle. The type of vacuum gage associated w/economy driving and such have damped needles- this is not what's wanted for diagnosing engine problems and for tuning.
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Old 08-31-2010, 06:11 PM
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That's the type of vacuum pump/gauge that I have. Have adjusted the hydraulic lifters three times. Last time I back them off as far as I felt comfortable. It's difficult to feel the resistance of the hydraulic lifters except when the are all the way down. Could be the new 5w20 oil, but I will try it again with intake manifold removed, so that I can see the lifters move.
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Old 08-31-2010, 09:21 PM
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Thats the wrong power valve which will really dump too much gas. The 6 was better but still not correct. You want 1/2 of your idle vacuum which is 11....you need a 4.5 because they dont have a 5 so go one lower

Timing at idle?
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Old 08-31-2010, 09:52 PM
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The list of Holley standard flow, single stage power valves available from Summit:

10
25
35
45
50
55
65
75
85
95
105

Holley high flow are available in 25, 35, 45, 55, 65, 85, 105. They also have 4-window and dual stage PV's. Possibly others.

Quick Fuel has a 70 along w/others, Summit has their own line of PV's as does Proform, AED, BG and et cetera.
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Old 08-31-2010, 09:55 PM
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Quote:
Originally Posted by sps_sac
Last time I back them off as far as I felt comfortable.
It's messy, but you may do better adjusting the lifters w/the engine running. You WILL know when there's lash- the clacking is unmistakable.

You can cut the center out of an old valve cover to keep some of the oil from flying around- but it's still messy.
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Old 09-01-2010, 08:36 AM
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About the Power Valve. Yes, I've seen the Holly Video but also seen several other online recomendations of selecting a power valve 1 to 2 below the intake vacuum, but yer it is running rich, but since I was trying to correct what appeared to be a lean backfire issue, thought I would try the rich recommendation to see if that cured the problem. It didn't, so I think the problem is with the valve lash adjustment. Engine is on a test stand, so economy isn't yet an issue and I will do a final carb tuning once I get it in the Falcon and out to the track. I've adjusted a lot of hydraulic lifters before, also the hot & messy method, but this engine is proving dificult. Last week I tried that method but when backing off the lifters I couldn't hear them clapping over the loud engine with all the noises from the electric water pump, the electric fuel pump and the mufflers. Maybe rollers cams are quieter; this is my first roller. The engine is on a test stand in my gargage and the Delta 50 Flowmasters are right there at the garage door. Now the other method: The springs on the lifters seem to be very soft giving little resistance when spinning the pushrods, so I'm going to pull the intake manifold so that I can see the lifters during adjustment. Hope to get the that done today.
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Old 09-01-2010, 10:59 AM
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Holley is correct, it's 1/2 idle vacuum in gear (for automatic) or in neutral for stick. On a stand you can't get into gear but if it's at 11hg manifold now....it'll drop in gear. 4.5 is my guess.

Also, you increased the accelerator pump size which is rarely done except double pumpers mostly. Not saying it's wrong but I think you're over fueled from both power valve and accel pump shot. You might have the idle mxture screws misadjusted?

The power valve should be closed at idle and open when you accelerate (when vacuum drops even more). At idle it's NOT in the idle circuit.

None of this might be causing your backfire though. If the backfires clears out I think the existing power valve will be an issue later.
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Old 09-01-2010, 06:40 PM
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A Holly Tech sugessted that I increase the squiter size from 31 to 37, but it didn't have the desired effect. I can fine tune the carb later, probabbly end up as it came out of the box. After pulling the intake manifold today and adjusting valves, I noticed that several of the hydraulic lifters were stuck at the top and wouldn't move down or adjust inside the lifter. Thereafter chatted online with a Comp Cams tech and he said they must be "pumped up" and that I need to take them apart to release the pressure. Did that, much better and they then had a total adjustment range of about 1 1/2 turns, set them all at 1/2 turn. However, I just got it back together, started it up, and it's the same as before! I guess now I need to check an see if they are pumped up again, which might be lifting the valves off their seats? Anybody had this problem before? Not sure if this is the problem. It is idling OK, an occasional pop, but if I hit the throttle quick, a big fire comes up the carb. This might have to wait until next week, going fishing up at Lake Davis by Portola, for the Labor Day weekend.
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Old 09-02-2010, 04:43 PM
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Just found this online: zero lash?

"Hydraulic Lifter Preload and Pump-Up- Hydraulic lifters are intended to make up for valvetrain dimensional differences as well as providing a self-adjusting method of maintaining valve lash, or rather the lack of. By setting the valvetrain so the lifter plunger is depressed slightly, the lifter is able to compensate for these differences, making a convenient hassle-free valvetrain set-up. For performance applications, lifter preload is not needed or wanted. As rpm's increase, the lifter has a tendency to bounce over the back of the lobe as it comes back down from the maximum lift point. The pressurized oil fills the lifter body to account for this bouncing. Eventually, after several engine revolutions (fractions of a second), the oil can completely fill the lifter body and the plunger will be pushed up to its full travel (pump-up). Higher oil pressures can amplify this problem. With the lifter pre-loaded, this can cause a valve to run off its seat and can cause piston clearance issues if and when pump-up occurs. By setting the valvetrain preload at “zero lash”, or just beyond, as felt by the hands and fingers during the adjustment process, lifter pump up is prevented and in most cases, the cam will rev higher. This adjustment process will typically end up with about .003” to .007” of lifter preload. Ford tech and tuning articles in the late 60's actually urged 'stock' class racers to run a positive lash of .001”-.003” on hydraulic cams."
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Old 09-02-2010, 06:09 PM
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I am still wondering about the valve spring height and/ or tension . I have caused hyd' lifters to pump-up by over-revving an engine but always was able to re-adjust themselves by just letting the engine idle for a few seconds , never more than half a minute .
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Old 09-02-2010, 08:21 PM
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The springs are the the standard which came with the AFR heads. I compared them to the Comp cams spring specs and they were very close to the same. I did shim them up just a bit to add a few pounds and even them all out. I think maybe the ramps on the aftermarket (Comp) cam are a bit steep and along with the roller, cause the lifters to pump up. Haven't had time to get back into the engine. Will likely be Tuesday before I can test this theory to see if it works.
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Old 09-02-2010, 08:34 PM
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I think the only running 3-4° initial advance is part if not most of the problem. you need to get that up into the 14-18° range. I am not familiar with your Mallory distributor so you may need to work with it(weights, springs, total stops) to ensure this doesn't put your total timing over 38°.

You are setting it with the vacuum advance hose disconnected, right??

Lifter pump-up only happens in an over-rev situation, not in normal operation, you are looking in the wrong area unless something is wrong with the lifters from the get-go like incorrect sizing of parts in them, incorrect assembly, etc. Oil pressure alone, no matter how high it is, can't cause lifter pump-up. High pressure/thick oil only slows the lifters return to normal lash after it has been pushed into an over-rev over-clearance situation. Valve spring pressure is way higher than oil pressure.
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Old 09-02-2010, 08:35 PM
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Quote:
Originally Posted by sps_sac
The springs are the the standard which came with the AFR heads. I compared them to the Comp cams spring specs and they were very close to the same. I did shim them up just a bit to add a few pounds and even them all out. I think maybe the ramps on the aftermarket (Comp) cam are a bit steep and along with the roller, cause the lifters to pump up. Haven't had time to get back into the engine. Will likely be Tuesday before I can test this theory to see if it works.
When you shimmed your valve springs did you consult a manual for installed valve spring height spec ? This is a critical measurement .Wondering if you shimmed them too much and bound up the springs and broke a spring or bent a push rod .
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Old 09-03-2010, 07:11 AM
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you need lots more timing at idle.

only 11 inhg of vacuum at idle is a problem with a cam that small. should be getting about 17 at 800 rpms.

low timing and improper fuel ratio will reduce vacuum.

I would run 15 initial, with 10 more from the vacuum advance on a manifold source (25 degrees at idle with advance).

then 35 to 36 total mechanical plus 10 from the vacuum advance (46 total).

your distributor should be full adjustable.
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