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Pilot Jets - 85 Suzuki dr 600 with flatslide accelerator Mikuni

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Posted by: Rabbit_Fighter---------------------

I was wondering if anyone who has jetted a few older single 4 strokes might enlighten me here? I bought it off ebay and have replaced, head bearings, clutch, fork seals, rectifier,,......just about everything that wears out. I'm not even sure if the TM flat slide was standard on this bike. The only manual available is in german, but I can read the part that says the original was a VM 38 Mikuni, which is a round slide.

Good motor though and pulls hard at everything above idle. I have set the valve clearances and fitted new needle and seat and have set the float bowl height. This carby has an airscrew below and in front of the carby. I've done the tune up method of warm up the bike,turn up the idle and listen to the point the idle is at its highest and use that as a means of setting the mixture. I'm finding that in order to stop it back firing on decell (lean?), I'm needing this air screw out about 2 and a half turns out which I understand is too many.

The only problem is it's damn hard to start cold. And rolling on from idle. Warm starting is one kick. Hesitation is most noticed if I go to top gear, or exit turns in high gear, and do a from just above idle roll on that big singles usually like. Once revs pick up it's strong. Seems to idle higher the warmer it gets.

I have a #25 Pilot fitted and have ordered a #22.5 and a #27.5. Fact is I'm not sure if I'm rich or lean. If it was a problem in the high rpm I could do a plug cut and see the condition. Will reading plugs from a vehicle at idle tell me anything if the problem is with the slow speed circuit? I would appreciate any comments from those with more knowledge than me of Mikuni TM 38 with accel pump on big 4 strokes.

Thanks

RF


Posted by: Ol'89r---------------------

RF.

IMO, you can do a plug check at idle or any other range of the carb. If the plug looks black or wet, you are too rich. You have to kill the engine while you are in the range you want to check. Keep the throttle in that range until the engine completely stops turning over. If you move the throttle before the engine stops, you will get a false reading.

A leaky exhaust system will also cause a backfire on decel. Make sure you exhaust system is properly sealed first.

Proper adjustment of the accelerator pump rod is very important on the TM carbs. If the accelerator pump squirts fuel before the slide opens, it will cause a hesitation. If it squirts fuel too long after the slide opens it will also cause a hesitation. Adjust the rod to that it squirts just after the slide opens.

A couple of twists of the throttle will sometimes help start a cold engine but to many twists of the throttle will flood it.

Just my . $.02 which is only worth $ .006 now, since our governor Benedict Arnold raised our taxes.


Posted by: Rabbit_Fighter---------------------

Ol'89r,

Thanks for that as I was wondering if I could check the plugs at any speed. I ended up using a smaller pilot jet and turned the fuel screw out enough so it doesn't backfire when you close the throttle off. I also had the fuel bowls set at the wrong height as I set it at 23.5 mm, which is the specs for a VM38, where the TM38 need to be set around 17mm.

Cold starting is easier, 5 kicks instead of 15. Will try your recommendation of giving it a few turns of throttle before cold starting . The next thing to check is the accelerator pump. It certainly works and I will check to see the timing of it as there is still that slight bit of hesitation as you roll on off low revs.
Not a lot of people here in Sydney are familiar with these old bikes. Seems they were a popular bike in Europe.
Thanks for the help.

RF


Posted by: spark250---------------------

good advice from Ol'89r. Setting the pump shot can be kind of a pain in the ass but it pays off when you get both the timing and duration set right.

I'm kind of surprised to hear that it is an accelerator pump carb though? I have a '96 DR 250 and it doesn't have one. But I guess maybe the bigger displacement engines had one?

Good luck!


Posted by: Ol'89r---------------------

Quote:
Originally Posted by Rabbit_Fighter
Ol'89r,

Thanks for that as I was wondering if I could check the plugs at any speed.
Cold starting is easier, 5 kicks instead of 15.
RF


Rabbit.

That method is not nearly as accurate at a full-throttle plug chop, but it will give you a rough reference point to start from.

Glad it's starting better.


Posted by: Rabbit_Fighter---------------------

Hi Guys,

The manual says this bike had the round slide VM carb so I'm assuming a previous owner put the TM flat slide in it with that accel pump.

I did a test at idle. Let it idle on clean plugs (this single has 2 plugs) for 10 minutes and they came out tan coloured, no black. Also tried a few handfuls of throttle before a cold start and that worked. First kick.

I think the accel pump is not being activated until about 1 third throttle. Shouldn't it be more like just as the slide starts to lift? I can see outside mechanical linkages and can observe that the plastic cam thing is not pushing down on the rod until the throttle is well and truly on.

I guess I need to pull the carby off (ouch) and observe the slide and when the pump rod is pushed down? I'm finally getting it sorted having bought it site unseen of ebay. I can see from putting a clutch in it that it had never been worked on and everything was tight. I love it but then, I love the way big single power on and prefer that sort of power over the screeamers.


RF


Posted by: Ol'89r---------------------

Quote:
Originally Posted by Rabbit_Fighter

I think the accel pump is not being activated until about 1 third throttle. Shouldn't it be more like just as the slide starts to lift? I can see outside mechanical linkages and can observe that the plastic cam thing is not pushing down on the rod until the throttle is well and truly on.

I guess I need to pull the carby off (ouch) and observe the slide and when the pump rod is pushed down?

RF


Rabbit.

The TM carb is an aftermarket carb. It was not OEM equipment on your bike. The VM carb is OEM.

Most of the jets in your carb don't start to work until air is introduced through the back of the carb, or slide. ( Idle circuit not included.) This causes a drop in pressure or a vacuum that sucks the fuel from the jets in the carb and mixes it with the proper air mixture before entering the engine.

If you adjust your ac rod to squirt fuel before the slide opens, you will have too rich of a mixture and when the slide opens it will flood the engine and most likely cause a bog or hesitation. Too much fuel before the jets actually start to work.

If you adjust your ac rod to squirt fuel too far after the slide is open, you lose the benefit of the ac pump all together. This will also cause a bog. Sounds like that is the way your ac rod is adjusted now.

Remove your carb and hook up a fuel line to it. Look in the back of the carb inlet and watch as the slide opens.The slide should open about 1/8 to 1/4 of an inch before the ac pump starts to squirt fuel. Make the proper adjustment of the ac rod and it will make a big difference.

Also check the position of the ac squirt nozzle. It has been several years since I have set up one of these carbs but if I remember correctly, the nozzle can be twisted in a way that the fuel will squirt sideways. Make sure the nozzle is positioned correctly so it squirts the fuel directly into the engine.

IMO, it's worth the time to set your carb up properly since the TM carbs make a huge difference in the way those old thumpers respond to the throttle when set up right.


Posted by: Rabbit_Fighter---------------------

Ol'89r,

That's very handy advice. The "bog" is not big, but not right and after sorting the jetting (somewhat) it's the accel rod. Starts first kick which is nice. I'd previously thought if I wound any gas at all I'd flood but seems a few hits of throttle before cold start, no choke, works perfectly.

I will pull it off. Not sure at this stage if I can alter the point that makes contact with the rod. I've ridden quiet a few of the 4 stroke singles and always thought this went extra for a 600 at most throttle openings. Thanks for the confirmation of this being non oem. It certainly works. Do you know what they usually fit the TM's to? Think this is a 36, though the oem was a 38.

Appreciate that you take the time to give detailed advice. Do you know how much the bike shops charge just to say g'day?

RF


Posted by: Ol'89r---------------------

Quote:
Originally Posted by Rabbit_Fighter
Ol'89r,

I will pull it off. Not sure at this stage if I can alter the point that makes contact with the rod. Thanks for the confirmation of this being non oem. It certainly works. Do you know what they usually fit the TM's to? Think this is a 36, though the oem was a 38.
RF


If I remember correctly, the rod is adjustable. You will be able to see how to adjust it when you remove the carb and observe the linkage between the throttle cable bellcrank and the ac rod.

The TM carb was an aftermarket or accessory carb. I'm not sure if it ever came as oem equipment. It was one of the first carbs offered with an ac pump. Compared to the carbs we have today, they are old technology but, they make a major difference in the way those old thumpers run compared to the stock VM carb.



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