AKSRXPilot
New member
In January I picked up a 1980 SRX 440 in very good condition with a brand new motor. Thanks to everyone here I have had great luck finding most of the information I need on this forum and have been able to get the sled working pretty good except for one lingering issue.
Fuel Delivery. The sled starts and runs fine for a few minutes. While driving at low or medium speeds when I apply lots of throttle the sled takes off then begins to backfire. The service manual says backfiring (Abnormal Combustion) is caused by lean mixtures and to check for dirty carbs and clogged fuel lines. Carbs are clean and fuel lines are not clogged. When I let the sled idle the fuel lines fill up completely with fuel. When I pop the cowling immediately after a backfiring session the fuel lines are dry but fill up after a few minutes of idling. I take off and it runs good again for about 30 seconds then begins to backfire.
Sounds like fuel pump but the pump looks good.
Piston wash and spark plug burn are nearly perfect. I'm running 95 octane and a 250 main for 20f days. As soon as it starts to backfire i shut it down.
After a little more research I figure out my 1980 SRX has a 1978 motor. 8G9 Cylinders and the 8JO heads. '78 motors had dual fuel pumps, 1980 motor had a single fuel pump.
I noticed last night that there are two nipples on the crank case for the fuel pump impulse lines. A previous owner tied both nipples together with a hose and T and then ran a single impulse line to the single fuel pump.
I am away from the sled for a few days so I can't do any experimenting but here is my theory. At low speeds the impulse signal coming from both cylinders is slow enough to make the fuel pump work properly. As soon as the RPM come up the pulses are to fast and the pump diaphragm never gets a chance to return for a complete cycle. The crank case impulses are to fast causing to fuel pump to quit pumping fuel.
All I need to do is go to a single impulse line and plug the other nipple.
Thoughts?????
Fuel Delivery. The sled starts and runs fine for a few minutes. While driving at low or medium speeds when I apply lots of throttle the sled takes off then begins to backfire. The service manual says backfiring (Abnormal Combustion) is caused by lean mixtures and to check for dirty carbs and clogged fuel lines. Carbs are clean and fuel lines are not clogged. When I let the sled idle the fuel lines fill up completely with fuel. When I pop the cowling immediately after a backfiring session the fuel lines are dry but fill up after a few minutes of idling. I take off and it runs good again for about 30 seconds then begins to backfire.
Sounds like fuel pump but the pump looks good.
Piston wash and spark plug burn are nearly perfect. I'm running 95 octane and a 250 main for 20f days. As soon as it starts to backfire i shut it down.
After a little more research I figure out my 1980 SRX has a 1978 motor. 8G9 Cylinders and the 8JO heads. '78 motors had dual fuel pumps, 1980 motor had a single fuel pump.
I noticed last night that there are two nipples on the crank case for the fuel pump impulse lines. A previous owner tied both nipples together with a hose and T and then ran a single impulse line to the single fuel pump.
I am away from the sled for a few days so I can't do any experimenting but here is my theory. At low speeds the impulse signal coming from both cylinders is slow enough to make the fuel pump work properly. As soon as the RPM come up the pulses are to fast and the pump diaphragm never gets a chance to return for a complete cycle. The crank case impulses are to fast causing to fuel pump to quit pumping fuel.
All I need to do is go to a single impulse line and plug the other nipple.
Thoughts?????
Throttle Junkie35
New member
Its just a theory, but I would guess having the pulse lines 't' into each other would probably cancel each other out and cause what you are describing.
I have never seen two pulse outlets power one pump...I also think this could be your issue.
AKSRXPilot
New member
Thanks guys I am leaning toward this being the issue with the fuel system. When i get back to the sled next week I will disconnect one side of the pulse signal and cap it off. I will report back with my findings.
I think I have the clutching fairly close. Engagement RPM is right at 5500. It pulls right to 8000 before it falls on its face and starts backfiring. I'm shooting for about 9200RPM at full shift out, at least thats what i think it should be.
I have found several stock Polaris P-85 style primary springs work well in the stock Yamaha clutch. The sled came with a pile of different weights and so far I am using the ones that were in the clutch which are the 8K2-17632-01-00 with rivets in both holes. They seem quite heavier than the 8K2-17000-10-00 and the 8G9-17000-10 both of which I also have.
I think I have the clutching fairly close. Engagement RPM is right at 5500. It pulls right to 8000 before it falls on its face and starts backfiring. I'm shooting for about 9200RPM at full shift out, at least thats what i think it should be.
I have found several stock Polaris P-85 style primary springs work well in the stock Yamaha clutch. The sled came with a pile of different weights and so far I am using the ones that were in the clutch which are the 8K2-17632-01-00 with rivets in both holes. They seem quite heavier than the 8K2-17000-10-00 and the 8G9-17000-10 both of which I also have.
Yamaha Nutz
New member
that would be your problem ..........motor is mot likely a 79 .........78 motor was a small stud (8mm cylinder nuts) and full 8G9 cylinders and head............79 and 80 motors had large cylinder studs (10mm) 8G9 cylinders and 8J0 heads ......79 had 2 pulse ports and 80 had one ........80 also had oil injection with the gear on the pto side of the crank....while all 78 -80 cranks will fit in all 78-80 cases
Last edited:
wayneo
New member
- Joined
- Dec 7, 2005
- Messages
- 21
I think you are on the right track, but also check your fuel tank vent line. If the line has a square end cut it can suck tight against the frame and not vent. Always cut vent line ends at an angle. I had a 79 SRX that did this.
YAMMIEGOD3:16
Active member
THATS A 79 MOTOR , JUST LIKE DESCRIBED ABOVE. I AGREE THAT IS YOUR PROBLEM. ITS A 79 MOTOR WITH THE 80 FUEL PUMP / SYSTEM. NEED TO ADD ANOTHER FUEL PUMP. ALSO 9200 TOO HIGH IF YOU RUNNING STOCK PIPES. 8800 TO 9000 is peak. 3:16 (yammie tony)