Slight miss/backfire at highway speeds?

[dabomb6608]

Ok so i made a longish trip today that was about 45min one way. Trip up there it ran great. It was early in the morning and cooler. Sat at my swim meet from about 7am to 1pm. Then drove to a restraunt. Ate and then back on the interstate. All is fine and dandy until about 10 miles from my exit. I start hearing this random kinda backfire/miss sound. Pretty quite but of course it was kinda hard to here with the top off at 70mph.

So I dont think much of it and drop my friend off at his house. Leaving his house i went to accelerate to 55-60 and whenever it got to highish rpms in third gear it started doin it again except this time it was more noticeable and if i gave it more gas to keep it out of OD it would get worse. Its not a LOUD backfire but more of a pop kinda backfire if ya get what im saying. And it only does it around 50mph and up in 3rd gear and OD. I can peg it from a stop and 1st & 2nd rev up just fine without a hickup or anything. I checked to make sure i didnt have a plug wire rubin the headers and getting burnt thru. And they were all attached so I was wondering what could cause this. Part that gets me is why only at those speeds/gears???

Truck details are 85 Jimmy 305 carbd with a 700R4. Engines got some bolt on upgrades nothin major. (headers, 350heads,intake, and no emissions.) Sorry for the long rambling haha

 

[Fordum]

Sounds like a timing problem. Vacuum advance would probably cause it to not show up at different throttle positions.

Quick check, reach back with the engine off and see if you can turn the distributor with moderate amount of twist.
Its possible that the lock bolt has worked loose and its self adjusting.

If its not loose, then maybe one of the advance weight springs has broken underneath it.

Before pulling it to see, I would pull the cap and check for a loose rotor or just general badness inside.

J.

 

[dabomb6608]

Ya that was my next plan of actions. Not so much the timing part but i will definitely check. Didnt even think bout that thanks I wouldve done a better look under the hood but it was boiling lava hot outside when i got home so you can only imagine how hot it was under the hood So i decided to instead work in the still boiling lava hot garage and modify/fab my roll bar i got for free

 

[StevoandaRedK5]

I would check the spark plugs before messing with the timing. Could have just fouled a single one and the timing is fine.

 

[dabomb6608]

Good idea. I wouldve thought that if it was only doin it at certain times though that a bad plug could be ruled out. Since it runs really good anyother time like around town. Either way i will check them as well just in case.
Thanks for the responses guys hopefully tomorrow b4 i go to my dads to finish the roll bar i can get a chance to check these things out. Goodnight and thanks again!

 

[dabomb6608]

Ok well this afternoon after we finished all the fabbing on the roll bar i checked the timing. It was loose so i tightend it up a bit. Took the cap off and everythin was fine. The only odd part was the centrifugal advance weights seemed to get jammed alittle so i put alittle lubrication on em and they worked fine. Took it out for a spin and still had the problem.

So got back to the house put some leather work gloves on(motor got warm really fast) and started pulling plugs. Pulled in this order 1,3,2,4,6,8,5,7. I figured i would pull the 4 closest ones first and hopefully find a bad plug. Well i didnt so proceeded to pull the rest. Had my dad check the gaps and they were all around .030 and the emissions sticker says .045 so he put them at that. Well as fate would have it number 7, the very last plug to be takin out, had a ton of whiteish carbon on the tip and around the electrode.

Took it to the wire brush and cleaned it up. Put the plugs back on, go for a spin, and problem solved. The rest of the plugs were just a white ish color and looked to be buring good with no deposits or blistering.

Thanks for the help on this guys much appreciated!

 

[Fordum]

Might want to keep an eye on that plug. Whitish carbon can be either oil or coolant. Or just a bad or too cool plug.
I used to remember all that stuff. Pretty sure oil burning causes the white stuff.

J.

 

[dabomb6608]

Ya it burns alittle oil at startup and less when it warms up. Can only notice it when you rev it up once its warm. It aint like its constant blue smoke out the tailpipes.
So we think that might be the cylinder with the worst oil getting past the rings or oil seal on the valves