Engine back-fire

If you convert over to R134a all the seals have to be replaced. The material used in the R12 systems is not compatible with R134a. I would convert over due to the price and availability of R12. As to the backfire, did you set up the valves in the new heads to match your cam?

Husker,

Thanks for the reply.

No, I did not do anything additionally to the valves (save stelite seats and a 3 angle job) or change the cam. The cam is stock. After discussing the project locally, I was told that swapping heads one for one should be no problem.

Thanks for the R-134 advice. Is it Ok to keep my rotary compressor? How cool will she work with R-134?

where in canada are you from?
im in canada also

I am in Ontario

What City, Province are you in?

Any advice for my problem?



andriyko

im in ont also
im near niagara falls
1. do a compretion test all cylinders

I didnt think that the valves on the stock 351 valves were adjustable. I know that they can be converted.

I reread your post agian. I had the same problem with hesitation on my 69 chevelle. It ended up being a bad vacuum advance on my new distributor. I also had to change the springs on the metering rods, which are the same thing as the power valve on the holley. I know that the old holleys had a problem with the power valves being ruined by the backfires. I would also check to make sure that intake and carb does not have a vacuum leak. ALso make sure that youir dist cap is sitting on tight

As for the backfire. Like was mentioned, a standard Holley after a backfire will damage the powervalve. (So you may wanna replace that). There is a anti-blowback piece you can install in the baseplate, that some of the "hotrod" stores like SummitRacing have.

Typically the backfire is a lean condition, so, for starters make sure that your initial setup is correct with the throttle and primary pump. (meaning if thats not giving a proper squirt thats a problem. You are opening the primaries for a shot of air w/o a good shot of fuel). Check to make sure that the "upside down" bolt on the throttle assembly is just-touching the pump lever under the front bowl. When you manually move the throttle you should get a good squirt of fuel. (and while I ramble, if you pulled your plug wires 2x check the order again. I've swapped 2adjacent wires, motor ran, and it did backfire)

First question/answer has a question that comes with it. Do you know if the timing chain has been replaced. Reason is, I'm thinking in 74' it was advanced offset 4degrees. Ultimately you wanna get about 34-36degrees full advance no matter what. (so its all relative). Some folks run around 8-10 initial advance to start with. Set it down there, and see if it at least moves upwards thru the rpm range to, lets say 3000.

On the A/C question. Sure, replace with the 134A if possible, MAKE SURE you get it totally converted and evacuated. The oils that are used with R12 and 134A are not compatable and will damage your system.

Good luck.....Bob

Thanks Bob

I am in the process of doing a thorough ckeck of the engine.

I have already replaced the Holley power valve.

The tune-up shops around here are telling me I have too much initial advance - the timing is set at 16 degrees. They think, like you, that I should be at about 9-10 degrees.

I will give that a go. Thanks for the advice kaamacat.

This reply is from Jack DeRyke. I will be posting responses from him temporarily. Thank you. Dave Adler

"Maybe, but the combination of parts you list will never allow tractor-like engine loads at 700 rpms. In fact, most carbureted 351Cs won't allow such low idle speeds- most idle at 1000or above. A few other things- engines built after mid-72 had 4 degrees retarded cam timing via the crank chain sprocket; most cam-chain kits allow multiple positions of the cam timing such as -4, straight-up & + 4 degrees. You might look at the cam you have & its timing position, maybe with a dial indicator. Also, stock Cleveland cam chains are really only good for maybe 20,000 miles before they're hopelessly worn & stretched. If you haven't yet, I'd suggest a new true-roller-chain & sprockets. Another undefined thing is, the fender tag initial timing of 16 degrees was for a low compression engine. Changing to a closed chamber head increases compression to 10:1 which is a bunch on todays gas. Too lg a carb (or a double-pumper thats not set up specifically for your egine) will also do this almost regardless of how you adjust things. If it really bothers you after checking the above, I'd buy an hour of dyno time & try various ignition & carb secondary tip-in settings; many Pantera owners find they make more power with less ignition timing.

Second question: I had replaced the A/C compressor with a rotary unit. The system was running R-12. My hoses (original) are now shot and need to be replaced. R-12 is no longer available in Canada (where I live). What can I expect if I replace the system with the new R-234 (I think that is what its called) or should I try to replace the hoses and try to get and stick to R-12?

Replacing R-12 with R-134a, your system will leak faster with the same flair-fitting brand-new hoses & cool less unless you change the separator can, the expansion valve under the dash and all the lines & fittings to barrier hose & o-ring style fittings specifically designed for R-134a. Changing compressor lube is also necessary; once all thats done, I'd think your A/C will work about as before for the area you live in. All the vendors offer a DIY kit of this stuff and most will do the installation."Jack DeRyke - PI Boss Wrench