Purpose of Trip:
I was having a difficult time getting my 38mm cast carb to run well on my 363 stroker motor. With the help of Corey at APT, we tried several different rods and other changes to address the issues, but to no avail. Something wasn't jiving. So, I drove to APT for a weekend trip to do some testing and dynoing to figure out what the problem was and get the engine running like it should.
Forewords:
I want to start off by saying I have great respect for Corey and APT. Their customer service and tech support has been fanstastic. He has worked very hard to help me out and help figure out the issue that several of us have been dealing with. Even though we are a tiny part of their customer base, they have still tried to do everything to make sure I (and others) are happy with the product. No other carb manufacturer has ever offered to help me in any way, much less not charge me for parts. APT has never charged me a dime for any of the extra parts that they sent me to try. Say what you want, but these guys are top notch in my book.
Time Invested:
I left out to head to Missouri from Tennessee on Thursday afternoon,took a vacation day on Friday, and arrived Friday morning. Corey and I spent from 9am to 11pm working, checking, wrenching, and dynoing Friday night. I spent a total of about 25 hours driving, spent a couple hundred dollars in fuel, and paid for 2 hotel rooms along the way. So please be respectful of any responses that you may have about this thread.
Testing:
test 1:
From riding and testing the carb at home, the issues we were battling was a gurgle at lower rpms like it was rich, but would then be lean and fall on it face in the upper rpms.
Before testing began, we disassembled the carb and inspected and cleaned it to eliminate an possible contamination issues. We started out testing my 38 cast carb with the L072 rod with the airbox and boot removed to eliminate any additional variables (since there was some question about the factory airboot possible causing a shrouding issue with the scoop that is made into the air side bell housing). This is unusual because the carb tends to follow the same linear pattern. Rich down low= rich throughout the whole power range, and vice versa. As soon as we began testing, the gurgling at lower rpms showed up. We took apart the carb again. We discovered that the nozzle (the part that the rod sits in that allows fuel through) orings were damaged and allowing fuel through the outside of the nozzle instead of through the middle where it was supposed to be. So we replaced the parts, assembled, and went back to testing. The gurgle was now gone. After a short amount of clicker adjustment, the engine was running strong and smooth. We then made some hard pulls in 4th gear from 25mph to 55mph and ran from low rpms to max rpm. The engine pulls all the way through very strong. It was finally running exactly like it was supposed to.
test 2:
We installed the stock airbox with stock airboot to test again to see if the airboot shrouding was an issue. We fired the engine up and made multiple runs. The engine was still responsive on the bottom up to midrange, but started falling on it face again in the mid to upper rpms. The step in the factory airboot does in fact cause an airflow issue with the scoop in the bell housing. The scoop is critical to design of the carb.
Main points from testing so far:
1. Nozzle was allowing raw fuel through at lower speeds causing gurgling. Orings damaged during installation or bore for nozzle not smooth enough due to carb casting. Bore smoothed and oring replaced. Gurgling no longer an issue.
2. Factory airboot shrouds scoop causing fueling issue at high rpms. Removed airboot and issue disappears.
3. Engine and carb perform as designed when no shrouding is present. Engine appears to really like the L072 rod at around 30 clicks.
test 3:
After we had the issues with the 38 cast carb resolved, we moved on to testing the 40mm billet for you guys that have the larger carb with the 363 engine. The tested the engine without the stock airbox and airboot since we confirmed from previous testing that the factory airboot causes airflow problems with the scoop.
The engine likes the Q05 rod the best at around 72 clicks. The engine ran really strong with the 40mm. No big surprise! lol.
Real World confirmation testing:
The next morning before loading up to leave, we took my R out to the open empty parking lot to confirm the same performance that we experienced on the dyno. We were still running without the airbox and airboot as the air conditions were still mostly clean. The engine ran flawlessly from top to bottom. The power delivery was strong and smooth, just like the dyno testing. The smart carb was now performing just as expected.
Conclusions:
With the 2 separate issues going on with the carb, I don't think we would have ever resolved them without making the drive out to APT. Too many variables. Due to the step in the factory airboot, it WILL CAUSE PROBLEMS WITH THIS CARB. This has to be addressed if you want full rpm range out of your carb. This is not a carb problem, this is a intake boot problem. This problem was hard to diagnose as well because almost no other intake boots have this step in the boot, except for the Honda 450R (not sure about which years). The permanent remedy for this issue will be to purchase: one for Troy Robl's need intake boot and airbox designs, LED's airbox eliminator setup, any other airbox eliminator setup, or fuel customs inc intake boot and airfillter combos.
I will be posted the dyno run videos as soon as they are ready. Note that they are NOT all inclusive. You can't see the numbers on the dyno. All you can gain from the videos is hearing the sound of the engine. If you have any questions, feel free to ask away and I will answer as best I can.
