1987 LT250R Quadracer carb jetting?
 

Looking for some help on jetting my Quadracer. Here's the deal!!! I have a 1987 Quadracer with a vm38 carb that seems to be sucking to much air. I want to match the gas the air but can't find anybody else running this carb on their machine. It is bored 060 over and have a DG header, Graydon Proline slipon, FMF Ram Valve with reeds, K&N cone filter (no more airbox), heavy porting, and run high octane (cam 2, Sunoco blue...whatever I can get a hold of). Anywho, I have a 132.5 jet in it right now. I also have a thunder powerjet installed but have it turned off right now until I get the right jet in it...then I will downsize and set my powerjet. This thing absolutely tears it up, but I just rebuilt it (low compression) and it seems to be way off. I can now hear it just SUCKING air through the filter. Still breaking it in but it seems so lean that I am afraid to keep breaking it in. I have a new plug in it and it looks brand new even after running it now for about 25mins while just at an idle and slow putting...remember still breaking her in. Does anybody else run a VM 38, and if so what jets do you run? Do they average in the 100's or 200's? Thanks for the help.

Are you running it with or without the airbox? If its removed, obviously you will need to go up in jet sizes. I outfitted my 89 Q-Racer with a 38mm TM flat slide carb. I don't recall the specific jet #, but I think it was around 260(ish) with premium gas/R50 blend at 32:1.

You might just have to do this one from scratch. Luckily, jets are cheap enough. It sounds like you could probably go up 4-5 jet sizes initially and see where that takes you, and then adjust as needed. You need to be running slightly rich anyways due to engine break-in. The sooner you hit a darker colored plug, the better. Of course, results may vary due to temp, elevation, humidity, etc...

Going back to the airbox... I remember reading a thread of QuadracerHQ about people having a difficult, if not impossible, time getting a consistent jetting due to lack of an airbox. I'll be darned if I can find that thread now, but... if i remember correctly, the fix was to run a short 3-4" piece of pvc tubing between the carb and air filter to straighten up the airflow before it enters the carb. Ignore this if this doesn't apply to your situation. But if it does, if you can google that thread... its really informative!

If those mods are all recent, you will certainly have to go up a few main jets from the previous setting. Whoever did your port work should be able to give you a decent starting point. Even without that, I would throw a real fat main in. Something that you are pretty sure will be a little rich and work your way back down.

With no airbox you are going to get a noticeable increase in induction noise, so that part is probably normal. Heck, just removing the lid from most race quads results in a good increase of intake sound.

All mods were done before I rebuilt it. The airbox is off and I barely have any room to put a piece of pvc on b/c of the rear shock. My cone filter is already almost touching it. From what I have seen main jets run on average about 4-5 bucks a piece, so I didn't want to be buying 8 of them to maybe get one that will be close....just needed a starting point, whether it be in the 100's or 200's. I do think that going with a big jet (180) to start is a good idea...won't hurt to run rich and then work my way down. Problem with that is I don't even know what to set the air/fuel screw at b/c the carb isn't stock. I am going to order a set of mid 100 jets for it and see what happens. I just can't see going up 10+ sizes on one rebuild. I do know that my compression went from 103psi to 168psi....maybe it is just sucking 10X's the air now and I will have to bump it up a significant size.

Good luck. I know firsthand jetting can be a PITA... but its very rewarding once you find the carb's sweet spot =) I'm not sure if this may help you or not, but, it'll save you some time from doing unnecessary plug chops.... When you fire up the engine, rev it, and the rpm's stay up there noticeably longer after you release the throttle... you are still too lean. A plug chop isn't necessary and you can go back straight back to increasing the jet size once again.

Pro-X sells individual jets for about half the price of Mikuni/Keihin. Always being on a budget, I went with these. I actually purchased a couple in sizes I already had in oem Keihin and the quad ran the same.

Engine is staying high after I rev it....thinking I might have an air leak, but thought it over for a couple days and realized it is way to lean...hence the jet problem. Never heard of pro-x but for some reason I have heard that an aftermarket jet and a mikuni jet are not really the same if they both say 160. Don't know how that is possible, but have heard complaints. Will def check out pro-x. Thanks.

You are right in that Dynojet is a popular aftermarket jet company. They sell complete kits and the jets are numbered differently. Pro-X sells/makes a number of engine parts such as pistons and clutch components. I have used their products with good results in the past. The jets were a perfect match and half the cost.

Ordered a quite a few Pro X jets from Rocky Mountain since they were so cheap!!!! Bought one Mikuni 150, and bought Pro X jets 160, 170, 180, 190, and 200. Pro X didn't make anything below 150, except for a 110...too low, plus I already have 80, 100, and 120 from a Mikuni Powerjet Kit. 132.5 is currently in it. Hopefully I will get a winner. Should be here tomorrow.

Good luck!