Exhaust restriction and Idle adjustment

[mikeffallon]

What is the relationship between back pressure and carburetor adjustment? After running for years with a seriously restricted exhaust system I corrected the problem and now the engine will not idle unless chocked at least 30%. During the time when the exhaust system was restricted, carbon build up caused 1 head failure on number 1 cylinder. But the engine idled fine and fuel consumption was OK (about 1.25 gals/hr). Relentless is a 1973 Ranger 37 with 2 blade 9" pitch X 18" folding prop. Engine has a 2 to 1 reduction gear on the aft section to handle the load. The engine also is equipped with electronic ignition and runs beautifully at all rpm ranges.
~Mike~

[sastanley]

Mike,

I would think that you now need to re-tune the engine from scratch given that you've removed the exhaust restriction. That restriction may have caused other problems (trash build up in the carb..carbon buildup in the cylinders, etc..)

What is the weight of your Ranger? 1.25 gal/hour seems a little high and could also indicate bad tune/running rich. I am burning in the neighborhood of .6 to .75 gal/hour @ 1,800 RPM on my 10,500# Catalina 30.

Do you have Don's engine manual? It is invaluable for troubleshooting these types of things. Mine is on the boat, so I can't be of much help here at work.

[msauntry]

I get around .75/hr in my big heavy A35, but his Ranger 37 trumps me. I'm careful to have a clean bottom and keep my speed well below hull speed since the burn rate goes much higher the closer you get to it. If the hull and prop isn't always clean, then this can also affect large boats disproportionately more.

I suspect a little tuning and cleaning can help. Carbon build up could be robbing some efficiency. Might be time to pull to carb off for a good look inside. Main jet should be a 21 fixed or a 24 with the adjustable main jet. Check with Don, but I think the 21 was the optimum size. Numbers are stamped on them. They are brass and are removed with a flat head screwdriver. Carbs have different jets for different rpm's. Idle jets handle idle speeds, and these might be a little clogged from your choke comment. Main jets handle higher throttle settings.

As far as relationship between backpressure and the carb... The engine is an air pump. What goes in must come out, and if your "output" is restricted then only that same amount can go in. The engine is restricted in its ability to move air, so you loose power.

[mikeffallon]

[QUOTE=sastanley;16465]Mike,

I would think that you now need to re-tune the engine from scratch given that you've removed the exhaust restriction. That restriction may have caused other problems (trash build up in the carb..carbon buildup in the cylinders, etc..)

What is the weight of your Ranger? 1.25 gal/hour seems a little high and could also indicate bad tune/running rich. I am burning in the neighborhood of .6 to .75 gal/hour @ 1,800 RPM on my 10,500# Catalina 30.

Do you have Don's engine manual? It is invaluable for troubleshooting these types of things. Mine is on the boat, so I can't be of much help here at work.

[Stanley (I assume it's Stanley) I do have Don's book and I will take your advice and do a complete tune up! The Ranger design Weight is 15,500 lbs. The boat gets weighed each year it is hauled and last time it dressed out at 17,500 lbs. Any recommendations for cleaning carbon from cylinders? Carb. Overhaul is planned for this weekend.]

[mikeffallon]

[QUOTE=msauntry;16498]I get around .75/hr in my big heavy A35, but his Ranger 37 trumps me. I'm careful to have a clean bottom and keep my speed well below hull speed since the burn rate goes much higher the closer you get to it. If the hull and prop isn't always clean, then this can also affect large boats disproportionately more.

I suspect a little tuning and cleaning can help. Carbon build up could be robbing some efficiency. Might be time to pull to carb off for a good look inside. Main jet should be a 21 fixed or a 24 with the adjustable main jet. Check with Don, but I think the 21 was the optimum size. Numbers are stamped on them. They are brass and are removed with a flat head screwdriver. Carbs have different jets for different rpm's. Idle jets handle idle speeds, and these might be a little clogged from your choke comment. Main jets handle higher throttle settings.

As far as relationship between backpressure and the carb... The engine is an air pump. What goes in must come out, and if your "output" is restricted then only that same amount can go in. The engine is restricted in its ability to move air, so you loose power.

[Thanks a lot for replying. A tune up is scheduled for this weekend as well as carburetor overhaul. The Ranger 37 is weighed each year it is hauled and last time it was 17,500 and some change. Is that comparable to your A 35? I had an A 30 for years and loved every minute I owned it.]

[msauntry]

I'll try to remember to ask the travel lift operator next time I'm hauled, but I should be in the neighborhood of 13-14,000 lbs. I've gathered from other forum users that the direct drive Atomic 4 is at the limit of its ability to move the larger boats (+34 ft) at hull speed or in choppy conditions. Someone suggested the reduction drive might be a solution. It makes sense that the larger boats won't be as efficient in our fuel burn rates, but I hope you see some improvements after your tweaking.

Best of luck!
-Micah

[Mark S]

Someone suggested the reduction drive might be a solution. It makes sense that the larger boats won't be as efficient in our fuel burn rates, but I hope you see some improvements after your tweaking.

I think the reduction drive would be helpful to deliver more power to the propellers on the larger boats, Micah's and my 13,000-14,000 lbs. and Mike's 15,500-17,500 lbs. I suppose more RPM for fewer propeller revolutions would use more fuel per hour. Easing the back pressure in Mike's boat should reduce his fuel consumption substantially. I think 1.25 gph is excessive under any circumstance.

Mark

[sastanley]

I would start with the 'base' settings using Don's manual and go from there. As we all know, the A-4 is a pretty simple engine, but if the few adjustments are out of whack, it may not run so well.

Good luck and let us know! :cool:

[mikeffallon]

I'll try to remember to ask the travel lift operator next time I'm hauled, but I should be in the neighborhood of 13-14,000 lbs. I've gathered from other forum users that the direct drive Atomic 4 is at the limit of its ability to move the larger boats (+34 ft) at hull speed or in choppy conditions. Someone suggested the reduction drive might be a solution. It makes sense that the larger boats won't be as efficient in our fuel burn rates, but I hope you see some improvements after your tweaking.

Best of luck!
-Micah

Micah
It just so happens that my A4 is equipped with a reduction gear housing. I understand that it is a two to one reduction which means that at 2000 RPM Engine Speed I'm spinning the prop at 1000 RPM. I took a lot of the very good advice I got from everyone on the forum and;
1. Tuned the engine (set the timing since I have electronic ignition installed
2. Cleaned the Carburetor
3. Replaced the Plugs and Wires
4. Adjusted the idle set screw

The result of all this a much happier A4 (and owner). Engine performance is about the same however. Max RPMs under load is 2100 and idles nicely at 900 rpm
Thank you for your help on this

[mikeffallon]

I think the reduction drive would be helpful to deliver more power to the propellers on the larger boats, Micah's and my 13,000-14,000 lbs. and Mike's 15,500-17,500 lbs. I suppose more RPM for fewer propeller revolutions would use more fuel per hour. Easing the back pressure in Mike's boat should reduce his fuel consumption substantially. I think 1.25 gph is excessive under any circumstance.

Mark

Mark
It just so happens that my A4 is equipped with a 2 to 1 reduction gear. It obviously help the engine push 16,000 lb RELENTLESS around I took a lot of the very good advice I got from everyone on the forum and;
1. Tuned the engine (set the timing since I have electronic ignition installed
2. Cleaned the Carburetor
3. Replaced the Plugs and Wires
4. Adjusted the idle set screw

The result of all this a much happier A4 (and owner). Engine performance is about the same however. Max RPMs under load is 2100 and idles nicely at 900 rpm
Thank you for your help on this

[mikeffallon]

I think the reduction drive would be helpful to deliver more power to the propellers on the larger boats, Micah's and my 13,000-14,000 lbs. and Mike's 15,500-17,500 lbs. I suppose more RPM for fewer propeller revolutions would use more fuel per hour. Easing the back pressure in Mike's boat should reduce his fuel consumption substantially. I think 1.25 gph is excessive under any circumstance.

Mark

Mark
Jury is still out on fuel burn rates but...
After taking I a lot of the very good advice I got from everyone on the forum and;
1. Tuned the engine (set the timing since I have electronic ignition installed
2. Cleaned the Carburetor
3. Replaced the Plugs and Wires
4. Adjusted the idle set screw

The result of all this a much happier A4 (and owner). Engine performance is about the same however. Max RPMs under load is 2100 and idles nicely at 900 rpm
Thank you for your help on this

[Mark S]

Thanks for the update, Mike. I'll be curious to see what your fuel burn rate is when you calculate it.

Mark