400SBC/300A choking questions

Propane, Butane, LPG, GPL, C3H8, C4H10
Billhilly
Posts: 27
Joined: Sun Apr 17, 2011 4:11 am
Location: New Zealand

400SBC/300A choking questions

Post by Billhilly » Sun Jul 03, 2016 3:04 am

Hi all,
I stopped in here a few years ago with questions about convertor mounting, and five years later I’ve actually got the truck going! Well on petrol anyway.

Brief history, I’ve got a 1972 GMC K2500 (3/4ton 4X4) that I’ve rebuilt over the last (many) years. Stock 350 was replaced (before I got the truck) with a 400 from a mid 70’s truck with an age appropriate Quadrajet on it. The motor sat for many years, and the only work I did on it was external. As far as I know the motor is basically stock, miles unknown, 330817 block, 462624 heads (second worst Chevy ever made?!) with a Performer Air Gap manifold, and Hedman full length headers.

I completely rebuilt the Quadrajet, and have worked through a few issues with it but it is running pretty good. From the get go I’ve run the 300A and 10” filter on it figuring I’ve got to tune the carb with the propane deal on anyway. I’ve done several thousand k’s and wanted to get the last bugs out of the carb, before running lines for the propane and getting that sorted.

I have installed an Innovate o2 sensor and have been keeping an eye on that. It cruises (100kph/2800rpm) almost spot on in the mid/high 14’s. The issue I had to deal with is that under moderate to heavy acceleration (2800rpm - 3300rpm) o2 dropped to low 12’s then fell to 10 and the motor actually started ‘stuttering’. I took the 300A off and gave the carb a couple of tweaks and took the truck for a run with my daughter in tow to watch the o2 gauge while I stayed between the fences! Funny thing was the drive was to see if my tweaks had done anything, but the revelation was it was cured! Cured by removing the mixer…… Put it back on and my ‘carb’ problems returned.

So yeah. I guess my question is along the lines of, if I can’t get enough air in when running petrol, it’s not going to be any better on gas? If that is a correct statement, then where to from here? I want to retain dual fuel. My books say a 425 is only flows 6% more than a 50&70. In real life do you think a 425 would be adequate? If no, what are some practical options? I have just finished rebuilding a VFF30, and Model L to run this deal. I've had 'experts' tell me I should be good to 5000rpm with this set up, but real world driving says otherwise. Just checked the Impco book, and it says 400CID doing 4000rpm equals 392cfm, so both 300A and 425 'should' work? Am I missing anything really obvious?

Sorry about the long post, but hard to convey without the details.


storm
Posts: 565
Joined: Wed Jan 10, 2007 10:10 pm
Location: NSW, Australia

Re: 400SBC/300A choking questions

Post by storm » Sun Jul 03, 2016 3:29 am

You have fitted the cable pull to the 300 and opened it up when running on petrol haven't you?
Every job is a self-portrait of the person who did it. Autograph your work with excellence.

Fuel flow requirements calculations viewtopic.php?f=5&t=1638

Billhilly
Posts: 27
Joined: Sun Apr 17, 2011 4:11 am
Location: New Zealand

Re: 400SBC/300A choking questions

Post by Billhilly » Sun Jul 03, 2016 4:07 am

Yeah, cable is there, but I am manually 'lifting' it open at the moment. It is sitting up on the cams fine. Opens around 16mm ish. I am guessing it opens the same amount with the diaphragm on gas?

kuchen
Posts: 79
Joined: Tue Oct 25, 2011 12:38 pm
Location: Stuttgart/Germany

Re: 400SBC/300A choking questions

Post by kuchen » Sun Jul 03, 2016 11:21 am

A manually lifted 300A is less restrictive than a 300A with the gas valve fully open. So the lifting mechanism does make sense on a dual-fuel setup.
In both cases, there is a depression caused by that mixer that does interfere with the Quadrajets secondary air flaps. Tweaking the spring load did the job for me.

Billhilly
Posts: 27
Joined: Sun Apr 17, 2011 4:11 am
Location: New Zealand

Re: 400SBC/300A choking questions

Post by Billhilly » Sun Jul 03, 2016 3:51 pm

Thanks kuchen! Funny because I backed the spring off a little bit the other day for my road test. I had been racking my brains as to what was going on with the carb.... I'll back it off some more and report back!

C3H8
Posts: 1000
Joined: Fri Sep 09, 2005 11:23 pm
Location: Winnipeg, Canada

Re: 400SBC/300A choking questions

Post by C3H8 » Sun Jul 03, 2016 8:03 pm

The 300 mixer has to have a cable attached to the cam to open the mixer. Once opened there is very little affect on the gasoline carburetor in most cases, however there was at least one model of the quadrajet it did affect. The cause was traced back to the bowl vents when the mixer was installed. When the mixer is installed it affected the breathing area above the rear vent creating a vacuum which changed the mixtures. You could attach a tube temporarily to the vent and route it through the mixer to check. On a few vehicles we actually drilled holes in the adaptor and attached the vent pipe so it could breath atmosphere and just added an external filter or routed it into the AC above the mixer.

C3H8
Posts: 1000
Joined: Fri Sep 09, 2005 11:23 pm
Location: Winnipeg, Canada

Re: 400SBC/300A choking questions

Post by C3H8 » Sun Jul 03, 2016 8:04 pm

Just got a response. Proquip has a repair kit in stock if you want to give them a call.

C3H8
Posts: 1000
Joined: Fri Sep 09, 2005 11:23 pm
Location: Winnipeg, Canada

Re: 400SBC/300A choking questions

Post by C3H8 » Sun Jul 03, 2016 8:05 pm

Oops wrong post! :oops:

Billhilly
Posts: 27
Joined: Sun Apr 17, 2011 4:11 am
Location: New Zealand

Re: 400SBC/300A choking questions

Post by Billhilly » Mon Jul 04, 2016 6:29 am

Thanks for your comments C3H8! I appreciate your time on this. There is only so much book learning you can do, and down here knowledge in this area is rapidly disappearing. I had to do book work tonight but will make time tomorrow to take the mixer off and take some pictures and report back. More questions to follow!

Billhilly
Posts: 27
Joined: Sun Apr 17, 2011 4:11 am
Location: New Zealand

Re: 400SBC/300A choking questions

Post by Billhilly » Tue Jul 05, 2016 5:54 am

Over view of filter/converter, and mixer in place.
20160705_202218 (640x480).jpg
300a manually opened
20160705_202500 (640x480).jpg
7042208 Quadrajet
20160705_202811 (480x640).jpg
Adapter front view
20160705_202942 (480x640).jpg
Rear view
20160705_202705 (480x640).jpg
Anything leaping out at you C3H8? Two things I noticed. I turned a few mm off the bottom of the adapter because I was looking at squeezing a larger diameter air filter in and wanted it to sit as low as possible. The adapter now sits down very close to the front vent tube. Back one doesn't 'look' to bad....but the way I had it clocked, one of the braces was very close to the rear vent.....

storm
Posts: 565
Joined: Wed Jan 10, 2007 10:10 pm
Location: NSW, Australia

Re: 400SBC/300A choking questions

Post by storm » Tue Jul 05, 2016 3:43 pm

The size of the filter isn't going to be helping.
Every job is a self-portrait of the person who did it. Autograph your work with excellence.

Fuel flow requirements calculations viewtopic.php?f=5&t=1638

Billhilly
Posts: 27
Joined: Sun Apr 17, 2011 4:11 am
Location: New Zealand

Re: 400SBC/300A choking questions

Post by Billhilly » Tue Jul 05, 2016 4:01 pm

Yeah my initial reaction was 'stupid filter', but then I took it off and had exactly the same results with just the mixer on.

C3H8
Posts: 1000
Joined: Fri Sep 09, 2005 11:23 pm
Location: Winnipeg, Canada

Re: 400SBC/300A choking questions

Post by C3H8 » Thu Jul 07, 2016 10:51 pm

Took a good look at the pictures. I reread your post and Kuchen's reply. Based on the fact that your showing really rich mixtures his theory is probably the better angle. The possibility I suggested would have resulted in leaner mixtures unless the vacuum created was so strong it pulled gas out the vent tube. Unlikely but possible. We had this happen on a venturri style mixer that was not vented properly. I would lean towards Kuchens reply because it sounds like you should just be reaching the point where the secondary air flaps should just be opening. If they aren't they would act like a choke drawing excess gas in through those huge sewer holes they had for secondaries. If you have a carb shop around ask them what would happen if you manually opened the air flaps just for a test run or if it is even possible.

I did see linkage in the third picture that either operates the secondaries or is a lock to prevent the air flaps from opening too soon. Have you inspected the adptor clearance to make sure it is not binding the linkage preventing the air flaps from opening.

storm
Posts: 565
Joined: Wed Jan 10, 2007 10:10 pm
Location: NSW, Australia

Re: 400SBC/300A choking questions

Post by storm » Fri Jul 08, 2016 1:49 am

C3H8 wrote:I did see linkage in the third picture that either operates the secondaries or is a lock to prevent the air flaps from opening too soon. Have you inspected the adptor clearance to make sure it is not binding the linkage preventing the air flaps from opening.
Good call C3H8
The linkage on the left of that picture, the one with the vacuum dashpot is for the choke and secondary pull off. If the choke is still on the secondary wont open.

Another thing to think about is the general tightness of all, but especially the rear 2, carby tops screws. That carb is from 1974 so it's not young by any stretch of the imagination and the rear screw holes have a tendency to elongate because of over tightening and force the top plate to bind 1 or both the rear air valve flaps. Add to that the adaptor possibly being to tight you are in a situation where no amount of vacuum will open them.

So check the linkages and also check the top screws are not causing any binding without the adaptor on, and then not causing any binding with the adaptor on.
Every job is a self-portrait of the person who did it. Autograph your work with excellence.

Fuel flow requirements calculations viewtopic.php?f=5&t=1638

Billhilly
Posts: 27
Joined: Sun Apr 17, 2011 4:11 am
Location: New Zealand

Re: 400SBC/300A choking questions

Post by Billhilly » Sat Jul 09, 2016 7:36 pm

Ok, so we're going with secondary airvalve spring adjustments first. And my first attempt to unwind it was a complete fail! If this is the answer, it's going to be a small adjustment.... I backed it off and ruined starting, still had overly rich acceleration, and richened up my cruise. Ran out of daylight/time last night, will try and make more time this afternoon. Will update shortly.
All good points about mechanical carb issues confusing things. I completely rebuilt the carb, and I believe all flaps, shafts, and linkages are good, including with the adapter on. I'll double check...again. Appreciate everyone's input.

Post Reply