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I run my 1970 LT-1 (11:1) on California 91.. No additives - no problems.
"Backfire" is not a symptom of low octane.. Pinging (pre-detonation) under acceleration is.
Fred
I run my 1970 LT-1 on 93 octane here in CT. Never a problem and no pinging under load
Agree with Polo. Stuck float or one of the floats set too high. Like Talvarez stated crud can effect the float's inability to close and at times this can even be intermittent crud is floating around the bowls. If the car is idling you should see no dripping from front/back venturi's if floats are okay. . I'd also check what power valve you are running. Can you take your carb apart? Oh...and let the car sit overnight before you pull it. Look down into the intake. You should see no small fuel puddles there if you pull it without releasing fuel. Just place it on a table with the metering blocks off. Use Carb Cleaner with a snorkle to clean out the bowls, all metering block crevices etc. You can visually inspect the power valve with a magnifying glass or just replace them with new from Summit. Once you see the power valve rating I'm sure some of the guys here can comment on it's number. Most carbs run a 6.5.
10:1 is pump gas compatible. I think you have other problems. Start by pulling the plugs and see how they look (especially differences between them) and while they're out do a compression test.
I agree with the general assessment that it's probably the carb but you need to make sure your engine is in good condition first. Also pump some gas out of the fuel pump into a clear jar and see what's coming from the tank.
So do I. It may be the gasoline but it's not the octane. You need to determine from what she suffers before you can get to the cause. Bad fuel varnish up the carb? Dirty fuel you accidentally introduced now clogging up the carb? Or an ignition or some other running problem, such as a vacuum leak or other defect that happened to rear its ugly head coterminous with your adding fuel. Many cars decide to give trouble unrelated to what you last did. That is the human condition.
Run a diagnosis. If you are not sure how and don't have help get out the shop manual (63 manual with 64 supplement, must haves) and turn to the engine diagnosis chart. Even a non-mechanic can use this to narrow down your problem to ignition, fuel, or something else.
I like to pull spark plugs and have a look. Pull one at a time, mark them so you know from what cylinder they came. Post a photo. There are some fellas here smarter than I who will help.
Dan
You guys are awesome. I am finishing up the garage (quckjack, mini-split system and garage door insulation) and will be back to diagnosing this soon, but you all have gave me a lot of very good advice and ideas to track down. I do have the 63 manual and 64 supplement. I really appreciate everyone who has chimed in and will update what I end up finding out. I absolutely love owning this car already.
@rgsGarage I'm not much help on the diagnosis, but I hope you get it figured out. I recently had my carb rebuilt by a guy in Catoosa. If you need his contact info, give me a shout.
@rgsGarage I'm not much help on the diagnosis, but I hope you get it figured out. I recently had my carb rebuilt by a guy in Catoosa. If you need his contact info, give me a shout.
Thank you, I changed out the fuel filter and the rubber lines to R9 lines on it but haven't started messing with the actual carb yet. I don't think it needs rebuilt, it looks new and I think it only has about 2700 miles on it. It is a Holley 750 double pumper. I haven't ever done much with carburetors before but I will be trying to check the float and giving it a tune here soon. One way or another I will get it figured out or find a garage here that can give it a good tune up. Thanks again, I'm hoping next spring you and I can meet up somewhere with our cars!
Just recalled 2 years ago I had a stuck float , really fat fuel smell at idle + low fuel pressure. When I took the carb off there were little black pieces of rubber inside. I went back to discover a faulty fuel line breaking apart inside by the intake. I could visibly see the rubber flakes inside the approx 8" rubber line. I blew the carb out with a snorkel carb cleaner , then replaced the fuel line & Summit fuel gauge..
Motor should run just fine on regular grade fuel. Did you look in the tank (easy to do) and see if there is some crud visible?
Octane boosters, race fuel etc not going to solve your problem. You've got some debris in the carburetor IMHO. Take the top off and have a look at the float bowls.
Just recalled 2 years ago I had a stuck float , really fat fuel smell at idle + low fuel pressure. When I took the carb off there were little black pieces of rubber inside. I went back to discover a faulty fuel line breaking apart inside by the intake. I could visibly see the rubber flakes inside the approx 8" rubber line. I blew the carb out with a snorkel carb cleaner , then replaced the fuel line & Summit fuel gauge..
This actually makes sense. The line between the fuel filter and carb was in pretty bad shape and could be the cause. That jet fuel I put in (octane booster) probably didn't help. I hope to get into the carb this weekend and see if I can figure it out. Thanks - Rob
Motor should run just fine on regular grade fuel. Did you look in the tank (easy to do) and see if there is some crud visible?
Octane boosters, race fuel etc not going to solve your problem. You've got some debris in the carburetor IMHO. Take the top off and have a look at the float bowls.
Nothing in the tank it looks pretty normal, but going to be checking for debris in the carburetor shortly, that seems to be the consensus on here. I will report back after I get into it. Thanks - Rob
I am leaning towards this as the problem now. Question:
This is a Holley 80508 750 Double Pumper carb. I am new to working on carbs. Is my best bet to take it off the intake and blow it out with carb cleaner on the bench, or trying to see if it can be fixed on the engine? I have a carb rebuild kit with the gaskets and plenty of U-Tube videos but the advice here is priceless. Thanks - Rob
Agree with Polo. Stuck float or one of the floats set too high. Like Talvarez stated crud can effect the float's inability to close and at times this can even be intermittent crud is floating around the bowls. If the car is idling you should see no dripping from front/back venturi's if floats are okay. . I'd also check what power valve you are running. Can you take your carb apart? Oh...and let the car sit overnight before you pull it. Look down into the intake. You should see no small fuel puddles there if you pull it without releasing fuel. Just place it on a table with the metering blocks off. Use Carb Cleaner with a snorkle to clean out the bowls, all metering block crevices etc. You can visually inspect the power valve with a magnifying glass or just replace them with new from Summit. Once you see the power valve rating I'm sure some of the guys here can comment on it's number. Most carbs run a 6.5.
I guess this answered my question. Thank you this is my project this week between Christmas stuff!
I am leaning towards this as the problem now. Question:
This is a Holley 80508 750 Double Pumper carb. I am new to working on carbs. Is my best bet to take it off the intake and blow it out with carb cleaner on the bench, or trying to see if it can be fixed on the engine? I have a carb rebuild kit with the gaskets and plenty of U-Tube videos but the advice here is priceless. Thanks - Rob
yes, easiest to pull it off the intake. If you have the correct rebuild kit, then disassemble and use carb cleaner ( and the included straw) to blow out all the small passages.
Wear eye protection, and do this well away from your car, and the carb cleaner could effect the paint and wax, etc.
I'll disagree in running a 10:1 IRON head motor on pump gas is OK. Yes it will run and maybe not hear any audible detonation but with most SBC iron heads the chamber is going to have some detonation.Throw one on a dyno or chassis dyno and run it on pump gas then 100+ octane gas and I'll guarantee more power as not detonating at all. It would not make it backfire unless it really detonated and broke a ring land out of the piston.
But I do second fresh gas and check the carb -old gas can varnish up real quick.
The smaller CID engines seem to be less prone to detonation than the larger ones. 283 gets away with more compression than a 383 for sure. Most of these discussions are about the 327 blocks and they do allow more compression than a 350 or 383. Many on here run a 327 with a claimed 11:1 compression that is in fact measured to be less than 10.5 with no trouble on Premium pump fuel. but these also run the more aggressive camshafts. 340, 350 and 365 HP engines.
Thank you forum! Stuck needle in the float it seems like it was. The carb is pretty new, but one of the two needles was stuck. The rebuild kit had spares so hopefully that fixes it. I do have another question below...
Stuck needle after the rebuild:
Put back together completely:
Now for the question. I have an extra piece that I am not sure if it found it's way into my pieces box, or if it actually goes somewhere I missed. So before I hook this back up to the car, I thought I would ask if anyone recognizes this piece and knows if I missed putting it on somewhere. It is black plastic.
