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Hank F.
Hank F., RV tech - emphasis on gensets
Category: RV
Satisfied Customers: 14406
Experience:  RV tech since the beginning of the millenium.
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troubleshooting an onan generator 7.5kw model# JB-3CR/1203/AD

Resolved Question:

troubleshooting an onan generator 7.5kw model# XXXXX serial# XXXXX 120/240v single phase 1800rpm 4 pull rotar. Electrical- LR-3927. I went over to troubleshoot for a customer of mine, the unit started on it's own & ran for 15 minutes. It transfered power properly & then backfired 3 times through the air cleaner & stalled. Over crank button tripped in tranfser switch I reset it & tried to re-start, enginge turns over but it will NOT fire. If it does fire, it runs for a short moment then backfires & then stalls. Any suggestion on the cause to this problem?
Submitted: 5 years ago.
Category: RV
Expert:  Hank F. replied 5 years ago.

Thank you for choosing Just Answer for the solution to your problem.
My name is XXXXX XXXXX I am going to assist you with this.


This may prove to be a difficult one to diagnose without actually being able to see it, but I'll do what I can.


However, I need just a bit more information from you.


1. What is the fuel source for this genset, gas or LP?

2. What is the model number of the transfer switch?

3. Was the overcrank tripped from the genset backfiring, or did it trip while you were trying to restart the genset?


Customer: replied 5 years ago.
c850752173 lp after first restart try bottle is 150 lb 1/2 full its in a foot of snow and has small fuel line 1/2 inch maybe i know its not right but it worked for 25 years till now
Expert:  Hank F. replied 5 years ago.

I still need to know the model number of the transfer switch itself.


When was the last time a good tune up was done to the genset, to include valve adjustments?



Have you had the gas pressure tested recently?


Where is the primary regulator located, at the tank or at the genset?


Are you getting good, consistent spark to the plugs?


Customer: replied 5 years ago.

complete tune done prior to calling you reg is at unit pressure is good 7 " water collunm

spark tester says good on spark and new plugs just gapped unit ran better than ever after tune up and this problem is why they called me originaly . i did tune and tested it ran fine and performed full load test and tranfer perfectly. then two days later on start up ran for 15 min. then backfired through exhuast. i dont have model for swith tags are not present any more. if it sound like bad valves it was 2 quarts low and last filter change was 2004 could this affect valves and if so should i sell him a new unit so i dont have to keep adj. valves every year and looking like i cant realy fix it?

Expert:  Hank F. replied 5 years ago.

The gas pressure is actually low, it should be 9"-11" WC.


As you know, backfiring is typically indicative of a lean fuel mixture or timing issue.

The low oil could be due to worn valve guides, or possibly a bad valve seal.

If this set is used for standby only, there is not reason the valves should need to be adjusted every year - although it certainly wouldn't hurt anything.


I suspect that most of your issues are going to be due to the low gas pressure - this would obviously also cause hard starting.

I would definitely adjust the gas pressure, and if it has been quite a while, adjust the valves.


I would think this will take care of it, but if for some reason it does not, let me know.


Hank F., RV tech - emphasis on gensets
Category: RV
Satisfied Customers: 14406
Experience: RV tech since the beginning of the millenium.
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