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  #21 (permalink)  
Old 06-04-2024, 01:54 PM
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What's the exact model number for your carb?
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  #22 (permalink)  
Old 06-04-2024, 02:00 PM
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Default Msd

hi,
Do you have an msd ignition box? if so that's the signs of it failing.
Perry
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  #23 (permalink)  
Old 06-04-2024, 02:07 PM
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Quote:
Originally Posted by eschaider View Post
What you have is a tuning problem and perhaps some parts that need replacement/adjustment, Brian.
Ed, no argument from me on the tuning problem. It is the solution that is proving elusive. Though Billy Andrews could have probably resolved this issue blind-folded....

But not as if I have been unwilling to throw $$ at it by deferring to experts. Once this issue began last Summer, I started to ask around for recommendations for a local shop. I was hesitant, but I followed the consensus feedback and took the car to a local shop claiming to specialize in 60s muscle cars. I didnt balk at the sizable quote for time on the dyno etc to tune the carb. Thing is, the shop never put the car on the dyno. They fiddled with the carb etc. I still got hit with a sizable bill, more than your estimate. And I didnt even get any time on the dyno.

So, believe me. I take your point. My incremental troubleshooting is far from ideal. At best, this is Plan C. Which is why I am so grateful to all here for the guidance. And patience. Hopefully something here also helps the next person that finds themselves in my situation.
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  #24 (permalink)  
Old 06-04-2024, 02:09 PM
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Quote:
Originally Posted by Grubby View Post
No vacuum connections in the carb shown in the photo.

John
i was hoping you would say that!
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  #25 (permalink)  
Old 06-04-2024, 02:12 PM
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Quote:
Originally Posted by patrickt View Post
What's the exact model number for your carb?
80805-1
Date code: 1177

My deciphering from the Holley website, the above info for my carb translates to ď950CFM ULTRA XP CARBURETORĒ
https://www.holley.com/products/fuel...rts/0-80805HBX

Date code translates to 117th day of 2017, or 27 April 2017
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  #26 (permalink)  
Old 06-04-2024, 02:40 PM
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MSD problem
Backfiring thru the carb is an ignition issue
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  #27 (permalink)  
Old 06-04-2024, 02:41 PM
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That's a pretty fancy carb. But a Holley is still a Holley and cracking open your secondaries is just the old school way to do it when you don't have an integrated idle bypass valve, which you have. 98% of all Holley drivability problems are the primaries not being set properly on the transition slot, the idle mixtures being off, the floats being off and the change in under hood temperatures that makes the air thinner in a car like your Cobra.

But, we could be completely wrong and you really have a bad spark problem. Maybe the MSD box, the coil, the pickup, who knows. The absolutely correct way to approach your problem would be to swap a known-to-be-wonderful Holley 750 Vac Secondary from someone's car at the local Cobra club on to your car and see if all of sudden the problems were gone. Is that a possibility in your case? It doesn't even have to be a Cobra club, an old Ford or Mustang club would do just fine.
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Old 06-04-2024, 03:02 PM
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Backfiring through the carb can also be a lean fuel mixture. But from what’s been posted, I don’t know why the carb would suddenly go lean unless the rebuilder changed the jetting or you do somehow have a vacuum leak. Also FEs are known to occasionally develop an intake gasket leak by sucking part of the lower gasket area into the runner and then it’s sucking air from the crankcase. Those are hard to diagnose other than just pulling the intake to inspect. You might be able to snake an inspection camera down the intake runners to look for any gasket protrusions into the port. You may get an oily combustion chamber too if it’s sucking oil in.

I’m still suspicious of timing though. If you can find someone with a dial back timing light and have them power time it at high enough rpm so that all the centrifugal advance is spun in. Did you receive any tuning instructions with it? I would guess you are looking for around 35/36 total advance (base timing and centrifugal).

I would also see if the accelerator pumps are responding immediately when you yank the throttle and there is a strong jet of gas in both primaries and secondaries. But be careful as you don’t want to be looking directly down a carbs throat when it backfires out the carb.

Good luck.
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Last edited by DanEC; 06-04-2024 at 03:06 PM..
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  #29 (permalink)  
Old 06-04-2024, 03:18 PM
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Quote:
Originally Posted by DanEC View Post

Iím still suspicious of timing though.
Well, I dunno... maybe. But his distributor pic looks fine. Better than mine, in fact. And my timing hasn't changed a single degree since it was set on the break-in dyno 20 years ago. In fact, I've never even cracked the hold-down nut on the distributor. For it to have changed in a few hundred miles, and the hold down bolt not being loose, is a long shot. But it wouldn't hurt to put a gun and check the initial timing and the all-in timing at 3500RPM.
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  #30 (permalink)  
Old 06-04-2024, 04:19 PM
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Quote:
Originally Posted by HighPlainsDrifter View Post
hi,
Do you have an msd ignition box? if so that's the signs of it failing.
Perry - my immediate answer was no. But then I went and crawled around the car once more to confirm. No ignition box under the hood - that much for sure. But thinking that maybe it might be on the other side of the firewall, i did find a box tucked up under the dash in the passenger footwell. But for the life of me, I could not find any markings on it. Hard enough to even snap a clear pic of it....
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  #31 (permalink)  
Old 06-04-2024, 04:35 PM
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Street Fire, a MSD product
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Last edited by sunman; 06-04-2024 at 04:38 PM..
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  #32 (permalink)  
Old 06-04-2024, 04:40 PM
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looks like MSD 5520 street fire ,CDI multi spark ignition box.
I was trying to post a picture. it took me so long sunman had already answered
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Last edited by rustyrims; 06-04-2024 at 04:42 PM..
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  #33 (permalink)  
Old 06-04-2024, 05:28 PM
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Quote:
Originally Posted by patrickt View Post
...The absolutely correct way to approach your problem would be to swap a known-to-be-wonderful Holley 750 Vac Secondary from someone's car at the local Cobra club on to your car and see if all of sudden the problems were gone. Is that a possibility in your case? It doesn't even have to be a Cobra club, an old Ford or Mustang club would do just fine.
The closest member of the New England Cobra Club I know is almost 200 miles away. Unfortunately...

I went to my local Cars & Coffee and asked around about local shops etc. And everyone pointed me back to the same shop that I tried last year. And they didnt seem to have much carb expertise. Unfortunately.

I have a buddy that knows how to use his timing light. When he is available, we can check the timing.

In the meantime, I will follow the Holley tuning instructions for the rebuilt carb.
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  #34 (permalink)  
Old 06-04-2024, 05:32 PM
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Quote:
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Street Fire, a MSD product


Quote:
Originally Posted by rustyrims View Post
looks like MSD 5520 street fire ,CDI multi spark ignition box.
I was trying to post a picture. it took me so long sunman had already answered
Thank you, both. This forum is the best. I post a picture of exactly what, I dont quite know. And within 20 minutes, multiple members respond to assist. Truly appreciated.
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  #35 (permalink)  
Old 06-04-2024, 05:43 PM
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if the gas in the tank is more than a year old I would drain that and start with fresh. reading your first post the gas might be from 2022. We have had race gas go bad in less than a year.
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  #36 (permalink)  
Old 06-04-2024, 06:14 PM
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Originally Posted by rustyrims View Post
if the gas in the tank is more than a year old I would drain that and start with fresh. reading your first post the gas might be from 2022. We have had race gas go bad in less than a year.
Great point. Not from 2022. But some of that gas in the tank could be approaching a year old.

Should be easy enough to drain that even I could do that...
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  #37 (permalink)  
Old 06-04-2024, 07:38 PM
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I hope it does not happen to you again but if it does backfire the best thing to do is to cover it with a blanket. A fire extinguisher makes a Hugh mess. I am confident you will get this sorted out. take your time
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  #38 (permalink)  
Old 06-05-2024, 02:24 AM
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Originally Posted by rustyrims View Post
I hope it does not happen to you again but if it does backfire the best thing to do is to cover it with a blanket. A fire extinguisher makes a Hugh mess. I am confident you will get this sorted out. take your time
Dry chemical extinguisher will make a mess of whatever it touches.

Halotron fire bottles are ABC, non-conductive and leave no residue.

https://www.h3rperformance.com/colle...-extinguishers
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  #39 (permalink)  
Old 06-05-2024, 05:14 AM
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Going back to your original post, I would go ahead and put the new plugs in. Iím kind of negligent in changing plugs regularly and last year my old 427 Corvette with a bunch of compression, cam duration, etc started running a little off. Finally put some new plugs in it and it was back to its usual self. Friend with a Hemi Roadrunner just had the same experience and his plugs werenít real old.
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  #40 (permalink)  
Old 06-05-2024, 05:14 AM
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Quinton Brothers. Williston. Pete Quinton made my 484 Keith Craft sing. He knows MSD and Holley's (Quikfuel). Once he set it up I never touched the Carb in all the years I owned it.(18) They also have a Dyno.

My E-M Cobra now sleeps in Paris, France
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