SRV runs into a brick wall (not literally)

A

AJSparko

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This was a project sled of my brother in law's and I did some trading for it. Part of the reason being he got so frustrated with it and moved on to another project.

Specs are it's an 87 SRV, bored .030, oil injection deleted, 38mm Miki roundslide with 370 main and 50 pilot, needle clip is bottom position, Boyeson dual stage reeds and DG pipe. Twist throttle and stripped down of a good 30lbs of weight it's gonna be a fun sled...if it ever runs right.

He's played with so many carb/jet combinations it's unreal. 38mm, 36mm rs...factory 44mm and 40mm butterfly, jetting up to 500 at one point on the mains and 30 on pilot...to the point where it's at now and just can't quite get it right.

What happens is it starts, idles ok, then the low/mid range is just flat, dead. It'll move pretty good to start on the snow, but you crack the throttle and it just has nothing there. Sputters a bit, and you really have to play with the throttle to let the engine pick up enough speed then KA-POW it's gone like a rocket. I was just messing with it yesterday and tried a couple of things...

Adjusted pilot down to 40, clip to middle, worse. Put back to other config. Drive it out of the shop, rolling along and slowly feed the throttle in, gets about 1/4-1/2 throttle and just bleh...nada, sputtering, keep feathering it a bit to pick up rpm/speed and BRAAAAAAP....hits that spot and runs out hard. Tried full throttle right out of the hole and it just falls on it's face.

Tried running a ground wire from motor to chassis, checked timing and it "looked" ok,(best I could see), new plugs, new wires, different coil....nothing has made a difference. I'm to the point now where I'm thinking of swapping in stock reeds but highly doubt that's it. Crank seals check out good, intake boot is brand new...oh and had Comet 108 on it, took that off and put stock clutch on, still no change.

CDI? Almost feels like an ignition/timing problem with the way it sputters but then at that one speed it does run hard and quick!
 
hmmmm glad to see you went back to the stock clutch... does that mean 102c? maybe it is engaging too soon and not letting the motor get into the powerband... i have fought and fought with carbs before and realized that it was the low rpm engagement the was making the motor bog.... what is it engaging at? if i had to guess i would say the 370's are pretty fat but i could be completely wrong... the reason i even mentioned that is, 540 vmax's (although completely different porting/ motor) take 290-310 mains when running 38's... i dont want to tell you that it is for sure rich on the the main but it is something to look into....
 
But aren't the 540 Vmax's dual carb? This is a 38mm single. No tach but engagement is up a ways, guessing around the 3000-3500 range. Threw on a known working CDI off a Phazer and it ran A LOT better. Weak spark in the mid maybe threw it all off so now jetting needs to be adjusted.

Turning idle air screw in all the way to 3 turns out makes no difference in idle so obviously the pilot jet needs to be bigger.
 
Throughout all this did it allways seem rich at low end then clear out at mid and run fine at WOT till you idled down. If so check the vacum suply line to the fuel pump. You may have a small leak in the vacum diaphram of the fuel pump , what happens is you start it cold with low fuel in tank it sucks raw gas through the fuel pump diaphram directly to crank case runs rich as you climb in rpm the vacum suply is not as strong does not pull as much fuel through the vac supply line it clears out runs fine till you return back to low rpm. I have been stumped by this a couple of times and gone through ,carb ,check comp, back to carb, check electrical, back to carb, checkreeds back to carb. symptoms could worsen with full tank and reduce as tank empties because of the hieght that the vac suply line enters crank case. the fuel pump with a small leak on vac side will still pump fuel well enough.When you pull vac supply line off there should not be any raw fuel in it.
 
The only consistent factor is that it's had this low/midrange "sputter" with poor throttle response. Just no "snap" to it like there should be. It almost seems like a bad timing issue, as if the midrange timing is out of whack. But once it hits that magic speed, there's definitely no issue. Then it has plenty of snap and acceleration. I checked timing at idle and the marks are dead on.

Fuel pump impulse line is dry, I've swapped the pump once already as a doublecheck.

Just the lack of throttle response alone is confusing me. I have a nagging feeling there's something that we overlooked and it's going to be a DOH! moment.
 
AJSparko said:
But aren't the 540 Vmax's dual carb? This is a 38mm single. No tach but engagement is up a ways, guessing around the 3000-3500 range. Threw on a known working CDI off a Phazer and it ran A LOT better. Weak spark in the mid maybe threw it all off so now jetting needs to be adjusted.

Turning idle air screw in all the way to 3 turns out makes no difference in idle so obviously the pilot jet needs to be bigger.
Click to expand...

So, the issues got better but are still the same?
Try tuning the carburetor in this fashion, get the sled to idle and respond from idle - meaning work from the pilot jet to top end, once the pilot circuit responds go to the needle and needle jet circuit (hopefully its a slide carb) once you get that straightened out then go to the mains.
Check the flywheel to see if there is any rust build up on the magnets, if there is, clean them off and you'll notice a huge diffrence.
 
Exactly. All the different carb/jetting combinations never changed the poor throttle response and midrange lag. Before I got it the ignition system was never really gone over, just constant carb changes.

Obviously since I noticed a considerable change with a good CDI that'll be replaced. I'm also going to take the time to check out the flywheel, pickups/magnets and verify the timing is correct. Coil, wires and plugs are good.

THEN, get into the carb so I'm not back tracking anymore. With a 38mm carb anyone have an idea for a good baseline of jets to run? Keep in mind this is a 540 bored .040 over with a DG pipe, Boyeson dual stage reeds, Uni pod. I'd rather deal with it being a bit boggy and taper back from there. I'm guessing a 60 pilot and 420 main to start. Any other suggestions?
 
If it was me I would put it all back to stock and then from there start changing one thing at a time ,starting with the carburetor, the .040 shouldn't make much difference in the carb specs.You may just have to many things giving you some problems,starting with one part at a time gives you the chance to dial it in before moving on to the next. Also try giving Reichards a call they used to sell duel Mikuni carb set ups and may have the carb specs for a single Mikuni ,I think Aaen also had a single 44 mm Mikuni carb set up for the SRV. Try giving these shops a call they should be able to help..
 
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i have had a simular prob twice and it was the fuel level in the bowl was to low, would idle fine had know throttle response in mids but could work it past by restricting air flow, then would go like hell and drop off again could check that, the other time some one trimed the needle jet portion that stands up into the carb bore, both these were on round slides
 
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