1979 Trailfire 340 carberator issues

[ve2hkw]

Hello,

Last fall, My brother got a 340 trailfire. It ran well but not for more than a minute. Through advice from a guy in the area, we determined that the engine seals were bad. So we redid all the engine seals and gaskets. Got it all back together and it ran for a bit, then it seemed to run rich all the time. It would start, run for about 10 minutes, and then die (about as far as you can get from the house....). We did numerous cleanings of the carb and redid the gaskets in it. We also got a new Fuel pump and fuel shutoff. (we were having problems with the float and fuel needle letting fuel leak by into the engine, that problem has been solved). Then spring happened, farming got in the way and I'm just getting back to it now. So I dissassembled the carb and realized that all the little components had numbers stamped on them. So I cross-referenced with the technical manual and they were all wrong..... So I have changed out the pilot jet (to size 35 from size 30) the Needle Jet from O-6 to P-0, the Jet needle from 6CEY6 to 6FL14, the Main jet from 200 to 210. I didn't spend the 26 dollars plus shipping on the slide valve, and the one that is in it is a 1.5. I have a chinesium one out of another carb that my brother bought last year off ebay (totally didn't solve our problems...) that looks to be like a 3.0 to a 3.5 but it isn't stamped....

All this to say that today with all the correct parts installed in a VM34 carb and all hitched up the engine with a new boot. It started after about 10-15 pulls (The carb had to fill with gas) I adjusted the idle and air screw all over the place (a little at a time) and couldn't get it to idle at all. I have to use the choke just a bit to keep it running at an idle. When I would go to full throttle it would just die, but if I added a bit more choke it would rev instantly (I have been running tests without the drive belt to avoid the sled rocketing out of the barn...) I have been keeping revs down to prevent a runaway and damage from that.

My question is basically I have gone from too rich to too lean, and I am looking for general advice from people who might have had experience like this. Should I begin by playing with the pilot jet (the air screw is turned in all the way and it still doesn't seem rich enough) or should I work on another component (like the slide valve or the needle jet) I checked the spark plugs and they went from chronically wet to dry so that is the basis of my information on the mixture (along with the choke helping it run rather than killing it).

Cheers from Southern Quebec,

Harrison Kyle

(as an introductory note as this is my first post here, my username is my Amateur (ham) radio callsign)

[jep_800]

Quite the quandary. Your needle, needle jet, main jet, and slide valve really don't come into play until you get to 1/4 throttle. (The slide can affect idle depending on where your idle stop screw is located... if it's cranked in, some fuel should get underneath the cutaway of the slide). So it all seems to be in how your pilot circuit is working. Another thing to check is to make sure the choke plunger is seated all the way down, and moves correctly when you pull the lever on the dash. This screws up your idle mixture if it isn't set properly. Since an air screw regulates Air, not fuel, I wonder if switching it to the higher number actually leaned it too much, and that's why the #30 pilot was in the carb? However, since you screwed it all the way in, it should get all the fuel and no air, so that's confusing.

Is your compression good? Did you get all the way to the crankshaft and inspect the center seals? It should still idle if your seals are good, you have good compression and spark, so the answer is someplace in the carb. Do you have the proper airbox on the carb? I don't know how those Trailfires are set up, but I bet it's got an airbox. Let us know what else you try.

JP

[ve2hkw]

Hi,

When we got the sled, it had good compression (we have a compression gauge and it was within the specs in the manual [110 psi I do believe]). We had it completely torn apart down to the crankshaft. My brother was the one doing that bit of work and he also was the one who bought the kit so I can't comment on the centre seal. The spark is good and other than the plugs running a bit rich for a while, they are new as well. There is an airbox, and it seems to fit well into the space in front of the fuel tank so I am confident that it is correct. It is missing a little elbow that bends from the airbox inlet to near a grate above the running boards. It doesn’t seem like that would throw things off too much but might be a factor.

I'm used to Marvel-Schebler carbs off of 2 cylinder tractors, so I'm still working out how to think about the Mikuni setup for the mixtures. I know turning the air screw out leans the mixture as it lets more air by and less fuel as a result. However I seem to misunderstand the pilot jet. I was under the impression that the number on the side was an indication of the size of the hole in the main bore. Thus a bigger number is a bigger hole. Going up one size would richen the mixture and going down one size would lean it. Thus going from a 30 to a 35 would richen things up. Perhaps this is a cause of my confusion.

Cheers,
Harrison

[jep_800]

Yep, it's been a while for me, but I had a sled with dual VM-34 carbs. It should be quite simple, as there are only so many things you can try to affect how it runs at idle speed.... I did have to play around with my pilot jets until I found the correct setup. It was different from the stock setup, but yet the manual tells you that if you don't have success with stock, change it either up or down until you get the result you are looking for. Do Trailfires have one carb?? Think it would be very simple if that is the case. Dual carbs always introduce the element of synching them together to get the cylinders to match fuel/air mixutures. Did you guys check the fuel pickup (check valve) in the tank? Sometimes those don't function as they should. When I take the tank off, I just replace them so I don't have a simple problem like that.

[ve2hkw]

Hi,

So I got to thinking about this issue one sleepless night and discovered the following, the float setup that was in the carb had the fuel level too low. Tonight I adjusted it according to the manual, and now I'm back to being too rich. I'm planning on adjusting the float arms a bit more to see what result I get and I'll post back here.

I also had a problem with adjusting the choke, so I'll need to address that when the carb comes back off the sled.

To answer your question, there is only one carb on the TrailFire 340. I think the 440 came with 2 carbs.

Also, we didn't check the check-valve, so it is something that needs inspection.

[Norton]

Hello Harrison,
Sounds just like what my friends 78 440 Cyclone was doing a few years back. He never fixed it! Sold it to another collector who is using it as a static display. It looked like new, just run like crap. We wanted him to install a new choke plunger (rubber seal on the bottom gets very hard over the years), new float valve (proper size), and check the timing (stator's can be weak too and engine still run). Good luck the Trailfire is a blast to ride!!

kb

[ve2hkw]

Hi,

So I scaled the float arm tab back a bit to lower the fuel level in the carb to no avail. (still too rich is seems) So now I'm going off on another line of inquiry, timing. I realized that the engine ran fine for the most part before we replaced the crank seals, and has ran terrible since then. I also went browsing through other parts of the technical manual and realized that we never properly timed the engine after reassembly. The engine reassembly guide section of the book is quite a few pages away from the timing section of the book. So I think the next step is to adjust the timing and then re-evaluate from there.