Looking for a little troubleshooting. Before the turbo200-4R upgrade, this q-jet pulled from idle to redline from any speed with no issues. After the turbo200-4R upgrade, the primary throttle shaft was changed to one with a TV cable pin and a 1" spacer was placed under the carb to get the clearance for the TV cable brackets. I did not do the shaft swap and the carb starts and idles just fine. He did re-epoxy the plugs as he said they were leaking. This guy generally knows what he's doing and may have even rebuilt the carb because he charged well over $100 to re-epoxy and swap the shaft. There is stutter/stumble between the 2 barrel and 4 barrel transition. I've got my vac gauge loaned out so I can't check it but the vac pull off on the side goes in upon startup and will stay if I compress it and hold my finger over the opening. The stutter happens with a sudden stomp or an easy work up from idle so I'm not thinking the accel pump. It is literally only at the transition point. Could the 1" spacer have caused some issue with mixture or does anyone suspect the swapping of the primary shaft altered my 2-4 linkage? I'll tear into it later but it ran fine before, now has the stumble. I've changed timing and have an electric pump so I don't favor a fuel delivery issue. Thanks for looking.
why did you change the timing? anyway, with someone sitting in the car and the engine off look down the secondary side of the carb and with the gas pedal floored make sure the throttle plates are both fully open. changing the linkage may have altered the throttle position. good luck
Timing was a lark. Just mentioned as ancillary information. We did have to switch to the Lokar throttle cable when the t200-4R went in. Also, it is a 71 big car carb, currently on a B4B intake with blocked heat crossover intake gasket so no choke. Could the 1" spacer have changed jetting needs at all? It used to be on the rich side with 72 jets. I'm going to check and make sure the rear hanger/rods are all connected as well. It is so hot I'm not feeling really motivated but that's the last major thing I need to fix besides a dash grounding problem.
"Could the 1" spacer have caused some issue with mixture or does anyone suspect the swapping of the primary shaft altered my 2-4 linkage?" Absolutely. I've tested all sorts of parts over the years, and modifications to parts that induced "stumble/hesitation/bog" when going quickly to full throttle. One of those items was spacers. A spacer placed on a dual plane intake can produce this problem. Especially if it is a fully open spacer. I've seen the same problem produced by cutting the center divider out of dual plane intakes. Some air clean assemblies will cause this problem as well, as will the K & N Xtreme lid, if it is placed too close to the top of the carburetor. I would suggest, if a spacer was used, that it have a full divider just like the dual plane intake. It should also be ported to match the intake under it. If the carb were rebuilt, it may not have received the later style accellerato pump assembly, which will also cause a stumble going to full throttle from lack of fuel delivery from the pump. Nothing rubber will live in this new fuel, and most "blue" seals are NOT ethanol compatable, as we have learned in the past few years since this new fuel came out......Cliff
Cliff, You entered these statements: If the carb were rebuilt, it may not have received the later style accellerato pump assembly, which will also cause a stumble going to full throttle from lack of fuel delivery from the pump. Nothing rubber will live in this new fuel, and most "blue" seals are NOT ethanol compatable, as we have learned in the past few years since this new fuel came out......Cliff What are the differences in the later style accellerator pump assembly? What correct replacement rubber pieces should we be using to live in today's gas?
The old style pumps were obsoleted by Rochester in 1975. They still show up in rebuild kits today. In 1975 Rochester shortened the pump seal "skirt" to make it thicker and more durable. To keep in against the pump bore, a "garter" spring was added under the pump. At the same time, they lowered the retainer position, and used a much stronger spring on the pump. A crimped on retainer was added with the stronger spring to keep the pump shaft from pulling out of the plastic pump body. The pump you are installing in any rebuild today MUST contain these upgrades, or you will be doing the job again. The addition of ethanol into our fuel recently has MANDATED the use of a high quality pump seal. Don't assume just because the seal on your pump is blue, that it is ethanol compatable. Most over the counter kits will have a "soft" blue seal the pumps, which swells up and either falls off the pump or quits working. There was a long/lengthy article by Lars on the Corvette Forum several months ago about this problem. The other item to mention about accl pumps is the springs. As soon as they get any rust on them, they loose their tension. They also get weak from decades of use. Always install a new style pump, with new springs and the latest flouroelostomer seal......Cliff http://www.cliffshighperformance.com/acc-pump.html
All of this just took place this year and I've only ran 1-2 tanks of gas through the car so far. The last rebuild I did on it prior was in about 2002 and it still ran great up until the spacer/trans/throttle shaft. I guess I'm going to have to bite the bullet and see what is what. Good ideas to look at the accel pump any way. I know I had tweaked the 2-4 linkage before so that it was absolutely perfect for my engine. Another question, what is the proper orientation for the rear blades at full throttle? I've seen them setup any number of ways and each has a (seemingly) valid reason. I've always leaned toward the slightly forward of vertical, as in not fully opened to vertical, where they create a wing like airfoil with the upper blades, pulling fuel out of the nozzles. This is how I had it setup prior to this winter. I will have to verify it is still this way. I've seen them vertical because that's the widest possible opening. I've seen them past vertical for similar reasons to my original theory but produce the airfoil on the back side. I've driven a car with this configuration and I swear it ran better not at full throttle travel. I think this one is definitely suboptimal.
Couldn't advise on the position of the secondary throttle plates. I've played with that deal on the dyno. In the testing I've done, full 90 degrees has produced the best dyno numbers in all cases. I've heard and read a few times where specific combinations have made more power slightly before center, or over center, etc. I can tell you this for certain, on single plane intakes, a Q-jet is going to need at least a 1" spacer, fully open and custom blended into the opening in the plenum, to make the best numbers. Not such a big deal on a dual plane intake, and in many cases we've seen a loss in power using a spacer, especially fully open and 4 hole. The best numbers I've made have been with a semi-open or fully divided spacer.....Cliff
So, I finally got off my lazy butt. The accelerator pump is fine. The thing that seems to have made the most difference is the rear throttle blades were going over center and opening a little early. I tweaked the linkage to have them come in a bit later and go vertical at full throttle. That seems to have it back like I like it! Thanks for letting me bounce ideas off you guys!
