Backfires under accelleration

[RZ_70coupe 11]

The car is a 69 with 302. Engine is internaly stock. Headers, C4, 4brl. original dist converted to Ignitor pointless.

 

About 2 months ago was having flooding issues with original 2brl. Changed to Holley 4brl (600, vac second, elec choke) and a dual plane intake. The car ran GREAT except for developing an oil leak. This was eventualy traced to a blown rear seal. During the oil leak investigation I repulled the intake in the event the gaskets were leaking there. While I had the distrib out I cleaned it with break cleaner, being careful to only clean the outside(hopefuly).

 

The next day going into work the thing began backfireing and could just keep 50mph. No power and backfireing like crazy. I checked the dist when I got it home and it looked fine. The vacume and mechanical advance were working.

 

I pulled the engine to replace the rear main and discovered the timing gear on the cam was SHOT. Guessed it had jumped. Reassembled the engine and it started and idled great. Took it around the block, very easy to insure all was well. Then tried a litle harder accelleration from a stop and BACKFIRE. So I checked the dist, timing, wires, etc. Can not see anything out of place.

 

The dist is original, but converted to igniter pointless. New cap and rotor, new wires, new plugs. Idles great. Prior to the backfireing it ran very strong and getting 17+mpg.

 

I'm stuck. It sounds like an ignition issue. I made sure the timming marks on new timming chain set were correct. Top one at 6 and bottom at 12.

 

Help:helpsmilie::helpsmilie::helpsmilie:

[LindenBruce 27]

While you had the distributor and intake off, did you happen to check the cam for lobe wear? A flat cam or broken valve spring and cause a backfire situation. Also, make sure your plug wires are routed correctly. On that point how old are the plug wires and are they factory 7mm or after market 8mm+? If you have two wires running right next to each other in the firing order that can cause inductive firing of the adjacent plug when the valve is still open. Bruce

[RZ_70coupe 11]

While you had the distributor and intake off, did you happen to check the cam for lobe wear? A flat cam or broken valve spring and cause a backfire situation.

 

Did not look 100% but the quick check looked OK.

 

Also, make sure your plug wires are routed correctly. On that point how old are the plug wires and are they factory 7mm or after market 8mm+? If you have two wires running right next to each other in the firing order that can cause inductive firing of the adjacent plug when the valve is still open. Bruce

 

New 8mm wires. using wire looms and routed same route as prior to backfireing.

[Print Dad 58]

Hello RZ_70coupe,

I would get a vacuum gage and check the vacuum.

I am thinking perhaps you have a vac leak on the intake.

 

Also - Holley carbs (which I love) don't like to back-fire.

It does not take much to blow the power valve inside the carb.

 

So I am thinking a vacuum leak has developed and perhaps you

may need to change out the power valve BUT check the vacuum first.

 

Print Dad

[RZ_70coupe 11]

Hey Print Dad,

Thanks for the reply. Guess I should have said the backfireing is from the exhaust and not through the carb. Sorry.

 

I'm going to get some carb cleaner tomorrow and with the car running spray around the carb. If there is a leak I should see the idel pick up.

 

I've not put a vacume gauge on the engine, but I do see the advance increase with the vacume line connected to the dist.

 

Total advance -Mechanical and Vacume - is about 50deg. Not sure what it should be with stock dist.

[Pakrat 1,043]

Double check the gap on the pertronix just to rule it out if nothing else.

[foothilltom 33]

Backfiring through the exhaust could also be traced to a rich mixture and/or exhaust leak. Are you certain the floats and fuel level are correct on the carb? Any chance your manifolds/headers are loose and/or the collectors to the exhaust are allowing in some air?

 

Just some random thoughts.

[Print Dad 58]

Hello RZ_70coupe,

FIRST - great to see Tom still helping here. You are the MAN.

 

Now back to the more serious issue.

 

Back fires can be caused by a number of things

1. - Incorrect timing

2. Valves not adjusted right - -worn etc

3. - Lean/Rich mixture - - bowl heights not right

4. - Vacuum leak

5. - Carb settings - - bad accelerator pump - blown power valve

- blocked passages

 

6. - Crossed ignition wires

7. - Bad/cracked dizzy cap

 

So many things - -BUT

This trouble seems to have started (back fire) after the intake removal.

 

I understand the car had trouble prior to that but I think you found the problem in the chain area.

 

Now you have a NEW situation which is the back fires.

 

I still think perhaps one of the intake gaskets has started to leak.

 

If you don't have a vacuum gage - - yes try the carb spray for certain.

 

Also - - double and triple check that the bolts on the carb (holding the carb to the intake) are tight. Make certain you didn't leave something loose.

 

Trust me - - we have all made simple goofy mistakes when working on cars.

 

So triple check the wires - -intake toruque. (I hope you followed the sequence and torqued to specs)

Check all the vacuum ports and lines. Make sure a cap did not fall off.

 

Look for simple stuff. After making certain all the simple stuff is right, I would then go into checking the carb.

 

Check the bowl heights - use the sight screws on the side of the bowls to be certain the fuel is just at the bottom of the hole.

Check to see that you are getting a good shot of fuel from the accelerator.

(Remove the air cleaner - -look inside the throat of the carb and work the

gas linkage. You should see a nice most of gas from both shooters.

 

I still think you will find that the problem is vacuum related.

 

Are yo certain you dropped the dizzy back into place correct? Most likeyl you did or she would not idle too well. Fords are pretty forgiving on the timing BUT it would not hurt to double check the timing as well.

 

Start with the simple stuff. Let us know

 

Print Dad

[Pakrat 1,043]

If you used the crappy cork gaskets on the front and back of the intake when replacing I would look thier first for a leak. They are total crap and should never be used, they could have easily been fine on idle and blown loose when you stomped it.

[yamahasrx700 14]

Burned exhaust valve

[RZ_70coupe 11]

Thanks to everyone for you help. Here is the rundown so far

 

Sprayed carb with cleaner - Nothing

Sprayed carb with quick start for better indication - Nothing

Checked PCV - Good

Checked for cross fire using inductive tach - Nothing

Checked for loose dist connections - All OK

Dismantled dist to check for sticking mechanical advance - OK

Used tach to check for mis-fire, ran to 2k RPM watched for change - Found 5 plugs were mis-fireing. RPM droping with throtel constent.

Pulled plugs - No visable issue, but found 3 set to .045 :( - Replaced each plug

 

Now runs much better. Only backfired twice when hard throttel at 65MPH. Still something there.

 

I'm using Autolite 45 plugs. Are these OK or are ther better?

 

Thanks, I'm getting closer

[RZ_70coupe 11]

Well another day another backfire :(

 

Sprayed Carb cleaner and Starter fluid anywhere I could reach that might have a vacume leak. Found the PCV connection into the carb was loose. Tightened and no change to running.

 

Replaced Pertronix back to points and condensor. No change in running.

Found the fuel line was sucking air at the out of the fuel pump. Replaced fuel line. No change to running.

 

Did compression check. one at 145 all otheres at 150-155psi.

 

So to me there could only be one of two things. PLEAST tell me ther is something else.

1. Mis adjusted valves. THis seems a bit of a strech, but never know. I had adjusted them befor the engine was put back in. Placing to TDC on compression, thghten rocker till no play on pushrod then1/4 turn.

2. Timming chain not on correctly. At most it could be out one tooth on the cam gear. I'm not sure how much of an issue this would be, but I'll put it out there.

 

Any more thoughts???

[RZ_70coupe 11]

Well another day another backfire :(

 

Sprayed Carb cleaner and Starter fluid anywhere I could reach that might have a vacume leak. Found the PCV connection into the carb was loose. Tightened and no change to running.

 

Replaced Pertronix back to points and condensor. No change in running.

Found the fuel line was sucking air at the out of the fuel pump. Replaced fuel line. No change to running.

 

Did compression check. one at 145 all otheres at 150-155psi.

 

So to me there could only be one of two things. PLEAST tell me ther is something else.

1. Mis adjusted valves. THis seems a bit of a strech, but never know. I had adjusted them befor the engine was put back in. Placing to TDC on compression, thghten rocker till no play on pushrod then1/4 turn.

2. Timming chain not on correctly. At most it could be out one tooth on the cam gear. I'm not sure how much of an issue this would be, but I'll put it out there.

 

Any more thoughts???

[LoneWolf2U 136]

Check total timing, it shouldn't have more than 42-46 all in at 2500.

My guess that with you having 50 your getting an overlap of spark causing the pop in the exhaust.

 

IIRC, turn the allen head screw in the dist vac adv counterclockwise to lower your total adv under vac.

[LoneWolf2U 136]

Check total timing, it shouldn't have more than 42-46 all in at 2500.

My guess that with you having 50 your getting an overlap of spark causing the pop in the exhaust.

 

IIRC, turn the allen head screw in the dist vac adv counterclockwise to lower your total adv under vac.

[RZ_70coupe 11]

STOP the PRESS'

 

Looks like I located the problem. Just remember KISS - Keep It Simple Stupid - Well the Stupid part comes into play here :scared:

 

When I went to the Holley 4brl, it had an elect choke. I grabed 12v from the feed to my pertronix. Sometime between then and now, not exactly sure where in the process, I moved the elect choke feed to the coil. THought was it was feed from a resistor wire and the lower voltage wold allow the choke to remain on a little longer this winter. Well I did not think about the coil and choke being in searies with the wire. THis would increase the load resulting in less voltage to the coil, which could cause weaker and/or missing spark. I just removed the choke and am reading 10.4v on the coil. Also noticed the coil is a 12v coil so I changed the feed to a solid 12v ignition feed. The car purrs. I went on a test drive and try as I might could not get it to fart out the exhaust.

 

One of those small changes that NEVER entered into my mind that I had done or that it would make any difference. THe fact that several issues appeared at the same time, Oil Leak, bad timming gears and all along it was something I did :scared:

 

I want to THANK EVERYONE for their aid and ideas in isolating this problem. Y'all are the best.

[RZ_70coupe 11]

STOP the PRESS'

 

Looks like I located the problem. Just remember KISS - Keep It Simple Stupid - Well the Stupid part comes into play here :scared:

 

When I went to the Holley 4brl, it had an elect choke. I grabed 12v from the feed to my pertronix. Sometime between then and now, not exactly sure where in the process, I moved the elect choke feed to the coil. THought was it was feed from a resistor wire and the lower voltage wold allow the choke to remain on a little longer this winter. Well I did not think about the coil and choke being in searies with the wire. THis would increase the load resulting in less voltage to the coil, which could cause weaker and/or missing spark. I just removed the choke and am reading 10.4v on the coil. Also noticed the coil is a 12v coil so I changed the feed to a solid 12v ignition feed. The car purrs. I went on a test drive and try as I might could not get it to fart out the exhaust.

 

One of those small changes that NEVER entered into my mind that I had done or that it would make any difference. THe fact that several issues appeared at the same time, Oil Leak, bad timming gears and all along it was something I did :scared:

 

I want to THANK EVERYONE for their aid and ideas in isolating this problem. Y'all are the best.

[foothilltom 33]

Congrats! Feels good, don't it?

[foothilltom 33]

Congrats! Feels good, don't it?

[Boogerschnot 12]

I was about to ask if you had gotten rid of the resistor wire for the pertronix but you got to it! Great job.