Suzuki SP370 Trouble

I’m new to this site but I’ve seen some other posts on here about SP370s that have been very helpful. My dad has had a ‘78 SP370 for over 17 years and he’s never heard it run. I wanted to get it running so I first cleaned out the tank which didn’t have that much rust but some really old gas. The I check ed the valves(they were in spec), set the points(they were a little off), and took off the carb to clean it out. The carb was pretty plugged up so I soaked it in chem dip over night and blew the parts out with air. When I put the carb back on, the bike started up on the 3rd kick. After a few seconds of reving it I tried to let it idle. It died and I couldn’t get it to start again. I’m stumped on what it could be. I have good spark, compression and the carb seems to be fine. Any ideas?

Now, does it still start OK, just refuse to idle?  That would really sound like the idle circuit, with the quite small holes and passages is still plugged.  The pilot jet has quite small holes in it too.  Is there an idle mixture screw and did you check it's position (how may turns out from bottoming) and put it back to the same place?  I have a can of that chem dip too and I know you're supposed to remove all rubber and plastic parts. Hopefully you did that. Try another dip and go at it with compressed air.

5 hours ago, motoxvet said:

Now, does it still start OK, just refuse to idle?  That would really sound like the idle circuit, with the quite small holes and passages is still plugged.  The pilot jet has quite small holes in it too.  Is there an idle mixture screw and did you check it's position (how may turns out from bottoming) and put it back to the same place?  I have a can of that chem dip too and I know you're supposed to remove all rubber and plastic parts. Hopefully you did that. Try another dip and go at it with compressed air.

It won’t start at all. I removed all the rubber and plastic pieces before I put it in the chem dip. The carb on the bike is very weird because it has the air screw like I’ve seen on other VM Mikunis but it has a second screw on the bottom side that controls the same circuit. I have the first air screw at 1.5 turns which is what it was at before I took it apart and the second at 2 turns which is what it was at before. I just don’t know why there are 2 screws for the idle circuit. I’ve never even seen another carb like this one.

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The bottom view pix is EXACTLY the same as the bottom of my '80 TS250 carb.  I have the shop manual for my '80 and the blow-up for the carb shows that screw but doesn't mention it anywhere, which really bugs me because mine idled poorly but no suggestion in the manual on how it should be adjusted.  After I found the problem of a leaky intake manifold (this is a 2T), then I just left the bottm one at 1 turn out.  I'm thinking the adjustments settings you have now wouldn't prevent it from starting.  How about doing a compression check?  I've had valves stick after sitting for a long time and that would answer that.  When you take the carb off now, Is the float bowl full of fuel - or half full?  It should be.  

The boot from the carb to the head is fine but the boot from the air box had a rip in it that I just fixed but I don’t think that would matter too much would it? And I haven’t used a compression tester but it I get the bike on the compression stroke I can stand on the kick starter without it moving and I weigh 170lbs. And the bowl is full when I take it off but no dripping from the overflow. I reused the old o rings because I couldn’t find a rebuild kit anywhere and I’m wondering if they aren’t sealing good enough and the vacuum is sucking past the mixture screws?

The screw on the bottom adjusts the fuel allowed on the idle circuit,the screw on the side adjusts air for same circuit.2.5 turns on fuel screw should get you close and 2.0 turns on air screw.The orifices inside of carb are very tiny and it doesnt take much to clog them.Are you missing the air cut off cover and diaghragm or just not installed in picture?These carbs are one fourth of the sets that were used on a few late 70's street bikes.Had a Kaw KZ 650 with carbs almost identical to yours except there were 4 of them on a rack.

Edited by bikefreak69
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Thanks for the info bikefreak but I’m not sure what you mean air cut off cover and diaphragm. But if you’re talking about what I circled in the picture then that doesn’t apply to this carb. None of those passages go anywhere.

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It bothers me that it wont start AT ALL now but at least did run before.  Have you checked for spark?  I'd still do a comp. check just to eliminate that.  If that's decent, I cant imagine why it wouldn;t run for a few seconds if you sprayed some starting fluid or just gas into  the cylinder. 

It has very good spark still and I’ll have to go pick up a compression tester to test it but it feels like there is a lot of compression. And here are the numbers on the carb if anyone knows what they mean. I thought this was a VM32 but the stampings don’t say so and the carb doesn’t measure 32mm either.

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32400 denotes 1978 SP370

Yes my bad for not paying attention to pic closer.Is it possible that your tank or fuel tank had some crud in it that then clogged carb passages again?I had that happen once with freshly rebuilt carbs and what i thought was clean tank and tap until I started and it ran good until next time i tried to start it.Carb is a Mikuni BS series 32 mm.

Edited by bikefreak69
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Well I took the carb apart again and there was a little bit of dirt in the bowl but not in the jets. I cleaned everything again and I’ll try it on the bike in a few days when it’s warm again. Hopefully it works out

I just skimmed through this:
Fuel filter?
When you open the fuel bowl drain,does it flow out good?
Also check the in tank pet cock screen,tank rust can clog it up.
Gets some ether to get it running.

The only filter is the petcock screen and it’s clear. The fuel flow is good to the carb. I’ve opened the drain a few times to check that when the carb was on the bike. I haven’t been able to mess with the bike because of the cold weather. Should be warm this weekend.

Try some starting fluid or gas in the cylinder.  If it runs with starting fluid, that tells us its a carb issue. 

And make sure that engine has good oil in it.

Edited by dirtjumpordie

Ok guys so I tried to start the bike again today and it wouldn’t go. Every time I pulled the plug out it was wet with gas. I tested the spark each time and it was really good. So I’m guessing it has to be compression. I’m gonna go rent a tester right now and see if it’s low.

Well I just tried compression testers from 3 different parts stores and none of them have the correct fitting for my spark plug. The plug is a NGK D8EA and the thread pitch is pretty fine thread. I guess I’ll have to find out what the thread is and buy a fitting. Man this is a hassle

“D” indicates a 12 mm x 1.25 size

The early DR/SP Suzuki singles were notoriously hard to start even when brand new.Plug is wet,and you have spark,it should run unless the spark timing is way off or the compression is way low-due to valve leakage or ring leakage.

3 hours ago, bikefreak69 said:

The early DR/SP Suzuki singles were notoriously hard to start even when brand new.Plug is wet,and you have spark,it should run unless the spark timing is way off or the compression is way low-due to valve leakage or ring leakage.

The cam and spark timing are good so I’m guessing low compression but I have to wait for the compression tester I ordered.

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