1994 Volvo 7.4 GL Stutters and Backfires at Mid RPM's

[jmsmieja]

Hey everyone,

I just went through the Holley 4150 750cfm on my VP 7.4GL, and put new 2.5 power valves in it. I also found that the jets were backwards from what I had read they should be (from the link below. I have 80378-1). I moved the 56 jet to the front and the 73 to the rear.

My problem is, now I seem to be running lean at the mid-range rpm's as I come off of idle and push to higher RPM's (~1000 to 2800). The engine stutters, the rpm's sway up and down by 500-1000 rpm, and I can hear it backfiring/puff through the carb. Does anyone have any ideas what I can do to fix this? This issue did not exist before I put the new power valves in and switched the jets around. Is it possible that the chart, in the link below, is not what I should be following? I've been told to try a bigger jet in the front (i.e. 65) and possible a 4.5 or 5.5 power valve. The boat did run really rich before, which is why I went through the carb in the first place. Now it seems too lean.

http://www.v8volvo.se/mekartips/tabeller/holley-specifications.html


Thanks for your help!

Jameson

 

[Bondo]

Re: 1994 Volvo 7.4 GL Stutters and Backfires at Mid RPM's

Hey everyone,

I just went through the Holley 4150 750cfm on my VP 7.4GL, and put new 2.5 power valves in it. I also found that the jets were backwards from what I had read they should be (from the link below. I have 80378-1). I moved the 56 jet to the front and the 73 to the rear.

My problem is, now I seem to be running lean at the mid-range rpm's as I come off of idle and push to higher RPM's (~1000 to 2800). The engine stutters, the rpm's sway up and down by 500-1000 rpm, and I can hear it backfiring/puff through the carb. Does anyone have any ideas what I can do to fix this? This issue did not exist before I put the new power valves in and switched the jets around. Is it possible that the chart, in the link below, is not what I should be following? I've been told to try a bigger jet in the front (i.e. 65) and possible a 4.5 or 5.5 power valve. The boat did run really rich before, which is why I went through the carb in the first place. Now it seems too lean.

http://www.v8volvo.se/mekartips/tabeller/holley-specifications.html


Thanks for your help!

Jameson

Ayuh,.... Welcome Aboard,... Goin' on that sentence, I'd guess ya gotta be jetted somewhere 'tween where ya started, 'n where ya are now,....

 

[Maclin]

Re: 1994 Volvo 7.4 GL Stutters and Backfires at Mid RPM's

I have seen that list before and thought it was accurate. The carb I have is on there, 80309, and when I took it apart the jets, even that the secondaries had staggered jets, and power valve matched those specs. It was virgin before I rebuilt it the first time.

Power valves with that low of a number will stay closed at much lower vacuum levels, that could be a factor. Also the 56's do seem too lean to me, but I am not going to 2nd guess that spec sheet.

Makes me wonder who changed the jets around and why in the first place!

The RPM fluctuation can be a power valve tuning issue, or actual vacuum leak. Maybe try a 4.5 pv in the front?

 

[Maclin]

Re: 1994 Volvo 7.4 GL Stutters and Backfires at Mid RPM's

JMS,

I remembered a little more about the list, the one I found a long time ago also showed what spring was in the vacuum actuator for the vacuum secondary models. I will try to find that list and see if it shows different jet numbers than this one.

 

[jmsmieja]

Re: 1994 Volvo 7.4 GL Stutters and Backfires at Mid RPM's

Thanks Maclin and Bond-o. I called Holley tech support and the guy I talked to said he thought 56's were too low as well, and that the carb at spec wasn't matched well to my engine and application. He suggested 68's, which I installed this morning and it runs like a top. I also bought a 6.5 power valve to put in the front, but wanted to try one thing at a time. I'm not sure if I'll put it in now or not.

Thanks again everyone.

Jameson